<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2023 Workiva-->
<!--r:68001b1b-4198-46f3-9c6b-c9d23d031f14,g:b22c4579-29c0-4556-966f-b015f670866c-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://mmc.com/role/CoverPage" xlink:type="simple" xlink:href="mmc-20230630.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://mmc.com/role/CoverPage" xlink:type="extended" id="if6d15a15bd4a4eb98ee76e6c83861826_CoverPage">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_78198c81-ec12-46b7-9239-24ff3cfb7dd5" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_DocumentType_78198c81-ec12-46b7-9239-24ff3cfb7dd5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_d697d4c3-d73e-4ac5-8c22-907104c0eae2" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_DocumentQuarterlyReport_d697d4c3-d73e-4ac5-8c22-907104c0eae2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_f9bf5f5e-68a8-4157-8458-b6643892fd5c" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_DocumentPeriodEndDate_f9bf5f5e-68a8-4157-8458-b6643892fd5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_d8b5392b-40c5-427b-8257-30b14e2d96fb" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_DocumentTransitionReport_d8b5392b-40c5-427b-8257-30b14e2d96fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_5815498e-7eee-4322-b33e-da248127a4fc" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityRegistrantName_5815498e-7eee-4322-b33e-da248127a4fc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_2716f149-3062-42b3-b012-8681e05da999" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityAddressAddressLine1_2716f149-3062-42b3-b012-8681e05da999" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_125391ff-f736-4e77-92f5-48fbc3fc338a" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityAddressCityOrTown_125391ff-f736-4e77-92f5-48fbc3fc338a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_e0aa0c62-16f7-41ef-b8a4-4cde23dbc873" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityAddressStateOrProvince_e0aa0c62-16f7-41ef-b8a4-4cde23dbc873" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_093440e7-87ca-46de-a94b-baf10491be52" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityAddressPostalZipCode_093440e7-87ca-46de-a94b-baf10491be52" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_86394f0b-ca94-49d3-a231-7f642931f374" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_CityAreaCode_86394f0b-ca94-49d3-a231-7f642931f374" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_68b826a2-27fd-4a2f-b096-1000ffc1fd71" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_LocalPhoneNumber_68b826a2-27fd-4a2f-b096-1000ffc1fd71" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_0f7ffc6f-b388-493d-9191-3ccbac08f7bd" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityFileNumber_0f7ffc6f-b388-493d-9191-3ccbac08f7bd" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_b33bbab9-8c56-4dc0-8108-101bc4582682" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityIncorporationStateCountryCode_b33bbab9-8c56-4dc0-8108-101bc4582682" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_472dd025-bccb-459a-b4fd-4ae50f54a1d8" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityTaxIdentificationNumber_472dd025-bccb-459a-b4fd-4ae50f54a1d8" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_5ba12b36-9197-4bec-af48-434db3188bd0" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_Security12bTitle_5ba12b36-9197-4bec-af48-434db3188bd0" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_3ebdfcdb-dfd7-42e0-a049-80ea5d556528" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_TradingSymbol_3ebdfcdb-dfd7-42e0-a049-80ea5d556528" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_b397a719-5128-4fd8-b802-6b17727ab8e1" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_SecurityExchangeName_b397a719-5128-4fd8-b802-6b17727ab8e1" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_1574e120-8d92-4d7c-9314-9ea3c9a26412" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityCurrentReportingStatus_1574e120-8d92-4d7c-9314-9ea3c9a26412" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_f51b1dbb-a5b3-47bb-a240-7feb869a9e16" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityInteractiveDataCurrent_f51b1dbb-a5b3-47bb-a240-7feb869a9e16" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_dffc66dc-16c1-43e5-8e2f-1bb887655ff6" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityFilerCategory_dffc66dc-16c1-43e5-8e2f-1bb887655ff6" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_8fcda9c8-d0fc-4c46-9a97-f3deb03a29ca" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntitySmallBusiness_8fcda9c8-d0fc-4c46-9a97-f3deb03a29ca" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_68f5453a-61fc-468b-a157-1651c9f9b996" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityEmergingGrowthCompany_68f5453a-61fc-468b-a157-1651c9f9b996" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_915a5c2d-605a-4720-9d62-7b22a2c9c0d8" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityShellCompany_915a5c2d-605a-4720-9d62-7b22a2c9c0d8" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_c5feed3d-19f9-4b9e-a4f4-028d32d17842" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_c5feed3d-19f9-4b9e-a4f4-028d32d17842" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_8ba74443-d5a3-41e8-b54a-7c0a1197dc0b" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntityCentralIndexKey_8ba74443-d5a3-41e8-b54a-7c0a1197dc0b" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_36c87dbb-f9bd-4add-ae02-e00b407f7c79" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_CurrentFiscalYearEndDate_36c87dbb-f9bd-4add-ae02-e00b407f7c79" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_c5416677-cdf1-4688-a992-3cb0941b76e4" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_DocumentFiscalYearFocus_c5416677-cdf1-4688-a992-3cb0941b76e4" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_524eafd2-2e51-4cba-9e86-3c0220ecb107" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_DocumentFiscalPeriodFocus_524eafd2-2e51-4cba-9e86-3c0220ecb107" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_12a2dc1e-3f3a-4874-a858-89431135a215" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_AmendmentFlag_12a2dc1e-3f3a-4874-a858-89431135a215" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitiesTable_3d517bcc-0615-451f-9270-6bdd751869a7" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityInformationLineItems_b0458045-c9d0-41cd-8e6b-8f965fc0c009" xlink:to="loc_dei_EntitiesTable_3d517bcc-0615-451f-9270-6bdd751869a7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsExchangeAxis_3d76f85e-1bed-4b53-8616-a448f3e2905b" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_EntityListingsExchangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntitiesTable_3d517bcc-0615-451f-9270-6bdd751869a7" xlink:to="loc_dei_EntityListingsExchangeAxis_3d76f85e-1bed-4b53-8616-a448f3e2905b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_3d76f85e-1bed-4b53-8616-a448f3e2905b_default" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityListingsExchangeAxis_3d76f85e-1bed-4b53-8616-a448f3e2905b" xlink:to="loc_dei_ExchangeDomain_3d76f85e-1bed-4b53-8616-a448f3e2905b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_d60dcbfa-bb92-4c9e-bd06-32ad9a879166" xlink:href="https://xbrl.sec.gov/dei/2023/dei-2023.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityListingsExchangeAxis_3d76f85e-1bed-4b53-8616-a448f3e2905b" xlink:to="loc_dei_ExchangeDomain_d60dcbfa-bb92-4c9e-bd06-32ad9a879166" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_exch_XNYS_f8dc395f-5377-4e89-86e7-09f40b1e65b5" xlink:href="https://xbrl.sec.gov/exch/2023/exch-2023.xsd#exch_XNYS"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_ExchangeDomain_d60dcbfa-bb92-4c9e-bd06-32ad9a879166" xlink:to="loc_exch_XNYS_f8dc395f-5377-4e89-86e7-09f40b1e65b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_exch_XCHI_548de4e3-ab37-4136-a416-ffc52bc83970" xlink:href="https://xbrl.sec.gov/exch/2023/exch-2023.xsd#exch_XCHI"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_ExchangeDomain_d60dcbfa-bb92-4c9e-bd06-32ad9a879166" xlink:to="loc_exch_XCHI_548de4e3-ab37-4136-a416-ffc52bc83970" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/ConsolidatedStatementsofEquityUnaudited" xlink:type="simple" xlink:href="mmc-20230630.xsd#ConsolidatedStatementsofEquityUnaudited"/>
  <link:definitionLink xlink:role="http://mmc.com/role/ConsolidatedStatementsofEquityUnaudited" xlink:type="extended" id="icc9ad52d15604c9baeb694744eef25ad_ConsolidatedStatementsofEquityUnaudited">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_63fb2781-3a0c-425d-b67c-e2500555f49a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_63fb2781-3a0c-425d-b67c-e2500555f49a" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_893bcba5-f7eb-42b4-8bf1-bc045ada82e6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_893bcba5-f7eb-42b4-8bf1-bc045ada82e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_aac5892e-9cbb-4802-a6be-08fa3a763662" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_aac5892e-9cbb-4802-a6be-08fa3a763662" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_StockIssuedDuringPeriodValueSharebasedCompensationandEmployeeStockPurchasePlanAndRelatedTaxDeficiency_8bb0abb8-40dd-41fc-95d1-c33a4f458fcc" xlink:href="mmc-20230630.xsd#mmc_StockIssuedDuringPeriodValueSharebasedCompensationandEmployeeStockPurchasePlanAndRelatedTaxDeficiency"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_mmc_StockIssuedDuringPeriodValueSharebasedCompensationandEmployeeStockPurchasePlanAndRelatedTaxDeficiency_8bb0abb8-40dd-41fc-95d1-c33a4f458fcc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestDecreaseFromRedemptions_63102a1b-9db8-4f15-8fb6-072e0d278b60" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MinorityInterestDecreaseFromRedemptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_MinorityInterestDecreaseFromRedemptions_63102a1b-9db8-4f15-8fb6-072e0d278b60" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_9102f7d6-6c84-4650-9175-c4ce410e674c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_ProfitLoss_9102f7d6-6c84-4650-9175-c4ce410e674c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsShareBasedCompensationCash_9f029790-1ba3-49f6-99d3-30fed1f5dbfe" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DividendsShareBasedCompensationCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_DividendsShareBasedCompensationCash_9f029790-1ba3-49f6-99d3-30fed1f5dbfe" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStockCash_e01698e8-691a-43d2-812f-6faff0b4b6e8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DividendsCommonStockCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_DividendsCommonStockCash_e01698e8-691a-43d2-812f-6faff0b4b6e8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_25b2c127-5bcc-42be-ad4e-2ae7435da4ec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_25b2c127-5bcc-42be-ad4e-2ae7435da4ec" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_d80ab719-4e11-4926-8516-bbb9ceff61a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_d80ab719-4e11-4926-8516-bbb9ceff61a8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_NoncontrollingInterestIncreaseFromBusinessCombinationsDecreaseFromDispositions_8f095632-6620-418d-b585-fd5f18b9e5ee" xlink:href="mmc-20230630.xsd#mmc_NoncontrollingInterestIncreaseFromBusinessCombinationsDecreaseFromDispositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_mmc_NoncontrollingInterestIncreaseFromBusinessCombinationsDecreaseFromDispositions_8f095632-6620-418d-b585-fd5f18b9e5ee" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_6347e912-4776-4d71-a716-0cf6b62c838d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_431f5958-b443-428f-9553-4f2614033267" xlink:to="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_6347e912-4776-4d71-a716-0cf6b62c838d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_a4c5d06d-0185-49bd-93be-665902925e4d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_CommonStockDividendsPerShareCashDeclared_a3b90552-6052-4a94-b7c4-80a0542ffa26" xlink:href="mmc-20230630.xsd#mmc_CommonStockDividendsPerShareCashDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_63fb2781-3a0c-425d-b67c-e2500555f49a" xlink:to="loc_mmc_CommonStockDividendsPerShareCashDeclared_a3b90552-6052-4a94-b7c4-80a0542ffa26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_c1828013-5d25-4a3b-8c54-2085389ee36c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_63fb2781-3a0c-425d-b67c-e2500555f49a" xlink:to="loc_us-gaap_StatementTable_c1828013-5d25-4a3b-8c54-2085389ee36c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_f2cdcb31-b8bd-4b9b-afdd-a7ebe761fac3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_c1828013-5d25-4a3b-8c54-2085389ee36c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_f2cdcb31-b8bd-4b9b-afdd-a7ebe761fac3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_f2cdcb31-b8bd-4b9b-afdd-a7ebe761fac3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_f2cdcb31-b8bd-4b9b-afdd-a7ebe761fac3" xlink:to="loc_us-gaap_EquityComponentDomain_f2cdcb31-b8bd-4b9b-afdd-a7ebe761fac3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_f2cdcb31-b8bd-4b9b-afdd-a7ebe761fac3" xlink:to="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_0a4010e0-3c7d-417b-ab5b-f2ffe52b62db" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:to="loc_us-gaap_CommonStockMember_0a4010e0-3c7d-417b-ab5b-f2ffe52b62db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_67dd85ce-a4ac-4c74-b71e-0a318f31e260" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_67dd85ce-a4ac-4c74-b71e-0a318f31e260" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_e1086501-2826-48e7-b395-c6ac11d2ca01" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:to="loc_us-gaap_RetainedEarningsMember_e1086501-2826-48e7-b395-c6ac11d2ca01" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_53dd304d-0380-451f-b634-474fb064aae5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_53dd304d-0380-451f-b634-474fb064aae5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_397118ee-14c3-4bad-a925-e60e39f0de1a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:to="loc_us-gaap_TreasuryStockCommonMember_397118ee-14c3-4bad-a925-e60e39f0de1a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_f01a04ba-89ef-4ebc-bc09-b3555ad0bd0b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_cffa058e-12ca-4cf9-b1ff-5a084ed9e3b9" xlink:to="loc_us-gaap_NoncontrollingInterestMember_f01a04ba-89ef-4ebc-bc09-b3555ad0bd0b" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/PrinciplesofConsolidationAndOtherMattersDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#PrinciplesofConsolidationAndOtherMattersDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/PrinciplesofConsolidationAndOtherMattersDetails" xlink:type="extended" id="ic3e3ea494cd545e284f3cce36f095ee2_PrinciplesofConsolidationAndOtherMattersDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue_c9d7ed9a-8910-493a-8a61-281139fb4a72" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue_c9d7ed9a-8910-493a-8a61-281139fb4a72" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_EquityMethodInvestmentLagPeriod_75a3a543-ec23-45da-aec5-49e171e129bd" xlink:href="mmc-20230630.xsd#mmc_EquityMethodInvestmentLagPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_mmc_EquityMethodInvestmentLagPeriod_75a3a543-ec23-45da-aec5-49e171e129bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnInvestments_d76ea3dc-b5e8-46d6-9302-6dd81b20acaa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GainLossOnInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_us-gaap_GainLossOnInvestments_d76ea3dc-b5e8-46d6-9302-6dd81b20acaa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_fe3ae068-6d61-42ea-a38d-dc585b5525c0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_fe3ae068-6d61-42ea-a38d-dc585b5525c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitPercent_f6bebb43-7894-4f95-b0d5-5cbd793068ae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitPercent_f6bebb43-7894-4f95-b0d5-5cbd793068ae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_8a509bad-65cc-4d09-8ccc-a5e3f9b4b518" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_UnrecognizedTaxBenefits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_8a509bad-65cc-4d09-8ccc-a5e3f9b4b518" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible_d89f691f-6723-4006-b290-843b1c473a9d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_us-gaap_DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible_d89f691f-6723-4006-b290-843b1c473a9d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_19ad3edc-54f9-4dbf-b9d6-0c30f1e18295" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleLineItems_62380a78-eb5f-47af-aa9a-245526940425" xlink:to="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_19ad3edc-54f9-4dbf-b9d6-0c30f1e18295" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_3b2c5fa7-fa12-4515-b6da-a4736e90ab31" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleTable_19ad3edc-54f9-4dbf-b9d6-0c30f1e18295" xlink:to="loc_srt_RangeAxis_3b2c5fa7-fa12-4515-b6da-a4736e90ab31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_3b2c5fa7-fa12-4515-b6da-a4736e90ab31_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_3b2c5fa7-fa12-4515-b6da-a4736e90ab31" xlink:to="loc_srt_RangeMember_3b2c5fa7-fa12-4515-b6da-a4736e90ab31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_3c4519de-273b-4dbc-8b36-cac03205e404" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_3b2c5fa7-fa12-4515-b6da-a4736e90ab31" xlink:to="loc_srt_RangeMember_3c4519de-273b-4dbc-8b36-cac03205e404" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_b1ff6af5-dd0d-4103-a371-a10e6449162a" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_3c4519de-273b-4dbc-8b36-cac03205e404" xlink:to="loc_srt_MaximumMember_b1ff6af5-dd0d-4103-a371-a10e6449162a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RevenueNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RevenueNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RevenueNarrativeDetails" xlink:type="extended" id="id432dacb764b4ffabd0b78d047e34a7e_RevenueNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_7b10e3d2-052e-4b13-839a-994c0b9123e3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RevenueFromContractWithCustomerThresholdPercentageOfTotalRevenue_4a4e47bf-63e2-4331-90d2-682dd44937cf" xlink:href="mmc-20230630.xsd#mmc_RevenueFromContractWithCustomerThresholdPercentageOfTotalRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7b10e3d2-052e-4b13-839a-994c0b9123e3" xlink:to="loc_mmc_RevenueFromContractWithCustomerThresholdPercentageOfTotalRevenue_4a4e47bf-63e2-4331-90d2-682dd44937cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_670fcddd-8685-4113-8e66-9e4df8b8bbfc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7b10e3d2-052e-4b13-839a-994c0b9123e3" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_670fcddd-8685-4113-8e66-9e4df8b8bbfc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_f2714f7b-a5db-4851-8a04-68e5eabdc320" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7b10e3d2-052e-4b13-839a-994c0b9123e3" xlink:to="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_f2714f7b-a5db-4851-8a04-68e5eabdc320" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_8c651766-9a16-4ce1-848b-eca4c1f6d849" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7b10e3d2-052e-4b13-839a-994c0b9123e3" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_8c651766-9a16-4ce1-848b-eca4c1f6d849" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_0e999fea-6c20-4757-a7b0-f7d82bc121ac" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7b10e3d2-052e-4b13-839a-994c0b9123e3" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_0e999fea-6c20-4757-a7b0-f7d82bc121ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsAxis_7cb70f1d-604e-4a0a-bb87-0275be8ad91b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0e999fea-6c20-4757-a7b0-f7d82bc121ac" xlink:to="loc_us-gaap_SubsegmentsAxis_7cb70f1d-604e-4a0a-bb87-0275be8ad91b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_7cb70f1d-604e-4a0a-bb87-0275be8ad91b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsegmentsAxis_7cb70f1d-604e-4a0a-bb87-0275be8ad91b" xlink:to="loc_us-gaap_SubsegmentsDomain_7cb70f1d-604e-4a0a-bb87-0275be8ad91b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_90d88d0c-f8d0-4439-85fa-cfebe1a7ebd1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsegmentsAxis_7cb70f1d-604e-4a0a-bb87-0275be8ad91b" xlink:to="loc_us-gaap_SubsegmentsDomain_90d88d0c-f8d0-4439-85fa-cfebe1a7ebd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MercerConsultingGroupMember_fb261dac-d023-47f1-853e-91c2fdfe08ab" xlink:href="mmc-20230630.xsd#mmc_MercerConsultingGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_90d88d0c-f8d0-4439-85fa-cfebe1a7ebd1" xlink:to="loc_mmc_MercerConsultingGroupMember_fb261dac-d023-47f1-853e-91c2fdfe08ab" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RevenueDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RevenueDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RevenueDisaggregationofRevenueDetails" xlink:type="extended" id="ica506d8d86a24f65b6c301d4c5b3ad5c_RevenueDisaggregationofRevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_1525777d-5ca7-45b5-82d7-70743c66d940" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_93029de3-7e6c-4035-81da-22d549e365eb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1525777d-5ca7-45b5-82d7-70743c66d940" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_93029de3-7e6c-4035-81da-22d549e365eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_InterestIncomeFiduciaryAssets_a311f894-52bf-4f98-b386-062e61066532" xlink:href="mmc-20230630.xsd#mmc_InterestIncomeFiduciaryAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1525777d-5ca7-45b5-82d7-70743c66d940" xlink:to="loc_mmc_InterestIncomeFiduciaryAssets_a311f894-52bf-4f98-b386-062e61066532" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_GainLossOnConsolidationOfBusiness_77424c0d-d1a7-4ff3-9be5-8b8c0e6a8054" xlink:href="mmc-20230630.xsd#mmc_GainLossOnConsolidationOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1525777d-5ca7-45b5-82d7-70743c66d940" xlink:to="loc_mmc_GainLossOnConsolidationOfBusiness_77424c0d-d1a7-4ff3-9be5-8b8c0e6a8054" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_2f811ff6-7d08-4042-88bc-7209ae709f3a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1525777d-5ca7-45b5-82d7-70743c66d940" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_2f811ff6-7d08-4042-88bc-7209ae709f3a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_0a4e1263-2e9b-4f80-9d28-6906f87dcd20" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_1525777d-5ca7-45b5-82d7-70743c66d940" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_0a4e1263-2e9b-4f80-9d28-6906f87dcd20" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsAxis_2fa79551-26c7-43c2-98b0-eb2ef8ca05db" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0a4e1263-2e9b-4f80-9d28-6906f87dcd20" xlink:to="loc_us-gaap_SubsegmentsAxis_2fa79551-26c7-43c2-98b0-eb2ef8ca05db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_2fa79551-26c7-43c2-98b0-eb2ef8ca05db_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsegmentsAxis_2fa79551-26c7-43c2-98b0-eb2ef8ca05db" xlink:to="loc_us-gaap_SubsegmentsDomain_2fa79551-26c7-43c2-98b0-eb2ef8ca05db_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_3bdab4f7-2338-4891-9f13-c285fba6ebf2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsegmentsAxis_2fa79551-26c7-43c2-98b0-eb2ef8ca05db" xlink:to="loc_us-gaap_SubsegmentsDomain_3bdab4f7-2338-4891-9f13-c285fba6ebf2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MarshGuyCarpenterMember_438d01b0-ad32-47aa-866d-ecfcf97537ea" xlink:href="mmc-20230630.xsd#mmc_MarshGuyCarpenterMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_3bdab4f7-2338-4891-9f13-c285fba6ebf2" xlink:to="loc_mmc_MarshGuyCarpenterMember_438d01b0-ad32-47aa-866d-ecfcf97537ea" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MarshInsuranceGroupMember_660ff057-2048-4038-89f9-0ebfb455e447" xlink:href="mmc-20230630.xsd#mmc_MarshInsuranceGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_MarshGuyCarpenterMember_438d01b0-ad32-47aa-866d-ecfcf97537ea" xlink:to="loc_mmc_MarshInsuranceGroupMember_660ff057-2048-4038-89f9-0ebfb455e447" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_GuyCarpenterReinsuranceGroupMember_4c602c90-4997-447f-af60-c36b11b629b8" xlink:href="mmc-20230630.xsd#mmc_GuyCarpenterReinsuranceGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_MarshGuyCarpenterMember_438d01b0-ad32-47aa-866d-ecfcf97537ea" xlink:to="loc_mmc_GuyCarpenterReinsuranceGroupMember_4c602c90-4997-447f-af60-c36b11b629b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MercerConsultingGroupMember_0eae2ee0-e7d3-4216-9c5f-fc230b5f8618" xlink:href="mmc-20230630.xsd#mmc_MercerConsultingGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_3bdab4f7-2338-4891-9f13-c285fba6ebf2" xlink:to="loc_mmc_MercerConsultingGroupMember_0eae2ee0-e7d3-4216-9c5f-fc230b5f8618" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_OliverWymanGroupConsultingGroupMember_af5a69d2-2cd3-4368-b171-0595f0775664" xlink:href="mmc-20230630.xsd#mmc_OliverWymanGroupConsultingGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_3bdab4f7-2338-4891-9f13-c285fba6ebf2" xlink:to="loc_mmc_OliverWymanGroupConsultingGroupMember_af5a69d2-2cd3-4368-b171-0595f0775664" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_eb0dd78a-6e07-4bdf-a3bb-92508904dd8f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0a4e1263-2e9b-4f80-9d28-6906f87dcd20" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_eb0dd78a-6e07-4bdf-a3bb-92508904dd8f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_eb0dd78a-6e07-4bdf-a3bb-92508904dd8f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_eb0dd78a-6e07-4bdf-a3bb-92508904dd8f" xlink:to="loc_us-gaap_SegmentDomain_eb0dd78a-6e07-4bdf-a3bb-92508904dd8f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_a7f3f176-8a32-42b3-9a13-3485a351f881" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_eb0dd78a-6e07-4bdf-a3bb-92508904dd8f" xlink:to="loc_us-gaap_SegmentDomain_a7f3f176-8a32-42b3-9a13-3485a351f881" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RiskandInsuranceServicesSegmentMember_8bfab5fe-9fa6-4b65-938f-bf7953b2e381" xlink:href="mmc-20230630.xsd#mmc_RiskandInsuranceServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a7f3f176-8a32-42b3-9a13-3485a351f881" xlink:to="loc_mmc_RiskandInsuranceServicesSegmentMember_8bfab5fe-9fa6-4b65-938f-bf7953b2e381" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ConsultingSegmentMember_fb702fe7-36c3-4d8d-811b-8fcff0955d60" xlink:href="mmc-20230630.xsd#mmc_ConsultingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a7f3f176-8a32-42b3-9a13-3485a351f881" xlink:to="loc_mmc_ConsultingSegmentMember_fb702fe7-36c3-4d8d-811b-8fcff0955d60" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_OliverWymanGroupConsultingGroupMember_f2c95497-b290-4f10-9227-e8b8dc87ee18" xlink:href="mmc-20230630.xsd#mmc_OliverWymanGroupConsultingGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a7f3f176-8a32-42b3-9a13-3485a351f881" xlink:to="loc_mmc_OliverWymanGroupConsultingGroupMember_f2c95497-b290-4f10-9227-e8b8dc87ee18" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SmallIndividualFinancialAdvisoryBusinessMember_08b103dd-a1d9-4379-97cd-403191bf3c79" xlink:href="mmc-20230630.xsd#mmc_SmallIndividualFinancialAdvisoryBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a7f3f176-8a32-42b3-9a13-3485a351f881" xlink:to="loc_mmc_SmallIndividualFinancialAdvisoryBusinessMember_08b103dd-a1d9-4379-97cd-403191bf3c79" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MercerUSAffinityBusinessMember_22834afc-a821-499f-a25d-7af8e43a03c7" xlink:href="mmc-20230630.xsd#mmc_MercerUSAffinityBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a7f3f176-8a32-42b3-9a13-3485a351f881" xlink:to="loc_mmc_MercerUSAffinityBusinessMember_22834afc-a821-499f-a25d-7af8e43a03c7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_34cb87ec-b242-4240-8c9c-0012267626c4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0a4e1263-2e9b-4f80-9d28-6906f87dcd20" xlink:to="loc_srt_StatementGeographicalAxis_34cb87ec-b242-4240-8c9c-0012267626c4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_34cb87ec-b242-4240-8c9c-0012267626c4_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_34cb87ec-b242-4240-8c9c-0012267626c4" xlink:to="loc_srt_SegmentGeographicalDomain_34cb87ec-b242-4240-8c9c-0012267626c4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_c0d89095-0e99-437a-a994-5e5a0e34efd2" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_34cb87ec-b242-4240-8c9c-0012267626c4" xlink:to="loc_srt_SegmentGeographicalDomain_c0d89095-0e99-437a-a994-5e5a0e34efd2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_InternationalMember_a03043f3-c43a-4943-bdd4-1a64545d2f67" xlink:href="mmc-20230630.xsd#mmc_InternationalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_c0d89095-0e99-437a-a994-5e5a0e34efd2" xlink:to="loc_mmc_InternationalMember_a03043f3-c43a-4943-bdd4-1a64545d2f67" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EMEAMember_f7443a07-c3e6-4732-8299-7118dca2e5bb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EMEAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_InternationalMember_a03043f3-c43a-4943-bdd4-1a64545d2f67" xlink:to="loc_us-gaap_EMEAMember_f7443a07-c3e6-4732-8299-7118dca2e5bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AsiaPacificMember_f1054c09-a613-45e3-8607-a3443e1ad3b1" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_AsiaPacificMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_InternationalMember_a03043f3-c43a-4943-bdd4-1a64545d2f67" xlink:to="loc_srt_AsiaPacificMember_f1054c09-a613-45e3-8607-a3443e1ad3b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LatinAmericaMember_a7bd1dde-da00-4755-942b-2cc268791dc4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_LatinAmericaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_InternationalMember_a03043f3-c43a-4943-bdd4-1a64545d2f67" xlink:to="loc_srt_LatinAmericaMember_a7bd1dde-da00-4755-942b-2cc268791dc4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_UnitedStatesandCanadaMember_35b0a649-40a9-4bcc-b03d-992e63fca1a6" xlink:href="mmc-20230630.xsd#mmc_UnitedStatesandCanadaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_c0d89095-0e99-437a-a994-5e5a0e34efd2" xlink:to="loc_mmc_UnitedStatesandCanadaMember_35b0a649-40a9-4bcc-b03d-992e63fca1a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_f7f0004f-e763-4ea2-be3d-734c28762295" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0a4e1263-2e9b-4f80-9d28-6906f87dcd20" xlink:to="loc_srt_ProductOrServiceAxis_f7f0004f-e763-4ea2-be3d-734c28762295" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_f7f0004f-e763-4ea2-be3d-734c28762295_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_f7f0004f-e763-4ea2-be3d-734c28762295" xlink:to="loc_srt_ProductsAndServicesDomain_f7f0004f-e763-4ea2-be3d-734c28762295_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_a53d5739-55a7-4e85-9065-54ed7af538a7" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_f7f0004f-e763-4ea2-be3d-734c28762295" xlink:to="loc_srt_ProductsAndServicesDomain_a53d5739-55a7-4e85-9065-54ed7af538a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_WealthMember_2a1f0218-47b7-4216-897d-bc41d2c4098e" xlink:href="mmc-20230630.xsd#mmc_WealthMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_a53d5739-55a7-4e85-9065-54ed7af538a7" xlink:to="loc_mmc_WealthMember_2a1f0218-47b7-4216-897d-bc41d2c4098e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_HealthMember_cf6b882a-491a-4787-90fc-25590ca404a5" xlink:href="mmc-20230630.xsd#mmc_HealthMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_a53d5739-55a7-4e85-9065-54ed7af538a7" xlink:to="loc_mmc_HealthMember_cf6b882a-491a-4787-90fc-25590ca404a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_CareerMember_848db5d5-8f5b-48c8-9faf-91d71bc3b20a" xlink:href="mmc-20230630.xsd#mmc_CareerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_a53d5739-55a7-4e85-9065-54ed7af538a7" xlink:to="loc_mmc_CareerMember_848db5d5-8f5b-48c8-9faf-91d71bc3b20a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_7a83c08e-b753-4f76-9b32-28ee16356c44" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0a4e1263-2e9b-4f80-9d28-6906f87dcd20" xlink:to="loc_srt_ConsolidationItemsAxis_7a83c08e-b753-4f76-9b32-28ee16356c44" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_7a83c08e-b753-4f76-9b32-28ee16356c44_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_7a83c08e-b753-4f76-9b32-28ee16356c44" xlink:to="loc_srt_ConsolidationItemsDomain_7a83c08e-b753-4f76-9b32-28ee16356c44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_f6dad5f0-3e83-4857-bf49-85095dc6a6e3" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_7a83c08e-b753-4f76-9b32-28ee16356c44" xlink:to="loc_srt_ConsolidationItemsDomain_f6dad5f0-3e83-4857-bf49-85095dc6a6e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_8a5ab453-1e0b-48a6-9b56-641b5a2e3067" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_f6dad5f0-3e83-4857-bf49-85095dc6a6e3" xlink:to="loc_us-gaap_OperatingSegmentsMember_8a5ab453-1e0b-48a6-9b56-641b5a2e3067" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails" xlink:type="extended" id="i0b09e3f8a34f4aa885c2968f2599453e_RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_2477b203-ff20-45a4-84f0-7df81288f177" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_2477b203-ff20-45a4-84f0-7df81288f177" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_b6e60445-1ccd-4cd8-8456-790b045e7ff5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_b6e60445-1ccd-4cd8-8456-790b045e7ff5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_40ff706b-a680-4d5d-b10e-ca07d37e138c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_40ff706b-a680-4d5d-b10e-ca07d37e138c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_746f622a-9a50-4b43-99f5-0e48e01d2188" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_40ff706b-a680-4d5d-b10e-ca07d37e138c" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_746f622a-9a50-4b43-99f5-0e48e01d2188" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails_1" xlink:type="simple" xlink:href="mmc-20230630.xsd#RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails_1"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails_1" xlink:type="extended" id="id156116ac38d495eaf192dc23deb3bfa_RevenueEstimatedRevenueExpectedtoBeRecognizedRelatedtoPerformanceObligationsDetails_1">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_2477b203-ff20-45a4-84f0-7df81288f177" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_2477b203-ff20-45a4-84f0-7df81288f177" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_b6e60445-1ccd-4cd8-8456-790b045e7ff5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_b6e60445-1ccd-4cd8-8456-790b045e7ff5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_40ff706b-a680-4d5d-b10e-ca07d37e138c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_856eed97-decc-4e55-a4e7-ec294b1b7d4a" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_40ff706b-a680-4d5d-b10e-ca07d37e138c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/FiduciaryAssetsandLiabilitiesNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#FiduciaryAssetsandLiabilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/FiduciaryAssetsandLiabilitiesNarrativeDetails" xlink:type="extended" id="i6b5a2cfac4564a79aadb9a95c660e1d9_FiduciaryAssetsandLiabilitiesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_0d3340de-79b4-441e-944c-0f6d7264acfe" xlink:href="mmc-20230630.xsd#mmc_FiduciaryAssetsandLiabilitiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_InterestIncomeFiduciaryAssets_e7f40769-0219-4869-a2f0-34463b87f9f8" xlink:href="mmc-20230630.xsd#mmc_InterestIncomeFiduciaryAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_0d3340de-79b4-441e-944c-0f6d7264acfe" xlink:to="loc_mmc_InterestIncomeFiduciaryAssets_e7f40769-0219-4869-a2f0-34463b87f9f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_UncollectedPremiumsandClaimsNetofRelatedPayables_0f86fc63-2320-4ab7-978d-92afb98a0b49" xlink:href="mmc-20230630.xsd#mmc_UncollectedPremiumsandClaimsNetofRelatedPayables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_0d3340de-79b4-441e-944c-0f6d7264acfe" xlink:to="loc_mmc_UncollectedPremiumsandClaimsNetofRelatedPayables_0f86fc63-2320-4ab7-978d-92afb98a0b49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_b48c4c51-7f7f-4c32-934a-be43630a59cd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_0d3340de-79b4-441e-944c-0f6d7264acfe" xlink:to="loc_us-gaap_Assets_b48c4c51-7f7f-4c32-934a-be43630a59cd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_FiduciaryAssetsAndLiabilitiesTable_0fdf9d06-b792-440a-b72e-1f6a492a9724" xlink:href="mmc-20230630.xsd#mmc_FiduciaryAssetsAndLiabilitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_0d3340de-79b4-441e-944c-0f6d7264acfe" xlink:to="loc_mmc_FiduciaryAssetsAndLiabilitiesTable_0fdf9d06-b792-440a-b72e-1f6a492a9724" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_361e7497-766e-4066-a605-c28675e54e66" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mmc_FiduciaryAssetsAndLiabilitiesTable_0fdf9d06-b792-440a-b72e-1f6a492a9724" xlink:to="loc_srt_ConsolidationItemsAxis_361e7497-766e-4066-a605-c28675e54e66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_361e7497-766e-4066-a605-c28675e54e66_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_361e7497-766e-4066-a605-c28675e54e66" xlink:to="loc_srt_ConsolidationItemsDomain_361e7497-766e-4066-a605-c28675e54e66_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_287d0fdb-555c-485c-9704-bb6d02c8ca67" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_361e7497-766e-4066-a605-c28675e54e66" xlink:to="loc_srt_ConsolidationItemsDomain_287d0fdb-555c-485c-9704-bb6d02c8ca67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_60c3e27b-c337-4dd6-83ec-c26b0e6d84d3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_287d0fdb-555c-485c-9704-bb6d02c8ca67" xlink:to="loc_us-gaap_OperatingSegmentsMember_60c3e27b-c337-4dd6-83ec-c26b0e6d84d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_267eaa4a-d93c-4901-8db4-d69bf86e467a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mmc_FiduciaryAssetsAndLiabilitiesTable_0fdf9d06-b792-440a-b72e-1f6a492a9724" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_267eaa4a-d93c-4901-8db4-d69bf86e467a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_267eaa4a-d93c-4901-8db4-d69bf86e467a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_267eaa4a-d93c-4901-8db4-d69bf86e467a" xlink:to="loc_us-gaap_SegmentDomain_267eaa4a-d93c-4901-8db4-d69bf86e467a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_665430ce-67f0-4aa2-b050-22646dc0adbb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_267eaa4a-d93c-4901-8db4-d69bf86e467a" xlink:to="loc_us-gaap_SegmentDomain_665430ce-67f0-4aa2-b050-22646dc0adbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RiskandInsuranceServicesSegmentMember_f1b9171d-ab5f-44f3-8cd0-ef2b894a6381" xlink:href="mmc-20230630.xsd#mmc_RiskandInsuranceServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_665430ce-67f0-4aa2-b050-22646dc0adbb" xlink:to="loc_mmc_RiskandInsuranceServicesSegmentMember_f1b9171d-ab5f-44f3-8cd0-ef2b894a6381" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ConsultingSegmentMember_bfdc0ca0-c728-4b02-9fc7-06a0d5a4e270" xlink:href="mmc-20230630.xsd#mmc_ConsultingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_665430ce-67f0-4aa2-b050-22646dc0adbb" xlink:to="loc_mmc_ConsultingSegmentMember_bfdc0ca0-c728-4b02-9fc7-06a0d5a4e270" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/FiduciaryAssetsandLiabilitiesReclassificationofFiduciaryAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#FiduciaryAssetsandLiabilitiesReclassificationofFiduciaryAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/FiduciaryAssetsandLiabilitiesReclassificationofFiduciaryAssetsandLiabilitiesDetails" xlink:type="extended" id="ic27259d24eed435093eb898a9f58bbc5_FiduciaryAssetsandLiabilitiesReclassificationofFiduciaryAssetsandLiabilitiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_b44ccc51-4d84-4311-a8c8-a7afd9dd59be" xlink:href="mmc-20230630.xsd#mmc_FiduciaryAssetsandLiabilitiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_66023ece-1de9-491f-becd-5bfeba8dc739" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_b44ccc51-4d84-4311-a8c8-a7afd9dd59be" xlink:to="loc_us-gaap_AssetsCurrent_66023ece-1de9-491f-becd-5bfeba8dc739" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_7a313cdb-333e-4cd2-a363-cc123cfe700a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_b44ccc51-4d84-4311-a8c8-a7afd9dd59be" xlink:to="loc_us-gaap_Assets_7a313cdb-333e-4cd2-a363-cc123cfe700a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_24018916-fc26-43e8-9e02-bcd53f1abe4a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_b44ccc51-4d84-4311-a8c8-a7afd9dd59be" xlink:to="loc_us-gaap_LiabilitiesCurrent_24018916-fc26-43e8-9e02-bcd53f1abe4a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_FiduciaryAssetsAndLiabilitiesTable_d7b31794-c219-46c9-8da1-8f29b1e924f9" xlink:href="mmc-20230630.xsd#mmc_FiduciaryAssetsAndLiabilitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mmc_FiduciaryAssetsandLiabilitiesLineItems_b44ccc51-4d84-4311-a8c8-a7afd9dd59be" xlink:to="loc_mmc_FiduciaryAssetsAndLiabilitiesTable_d7b31794-c219-46c9-8da1-8f29b1e924f9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_c0efa266-98a7-42da-8740-f3a7f7ffa3d9" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mmc_FiduciaryAssetsAndLiabilitiesTable_d7b31794-c219-46c9-8da1-8f29b1e924f9" xlink:to="loc_srt_RestatementAxis_c0efa266-98a7-42da-8740-f3a7f7ffa3d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_c0efa266-98a7-42da-8740-f3a7f7ffa3d9_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_c0efa266-98a7-42da-8740-f3a7f7ffa3d9" xlink:to="loc_srt_RestatementDomain_c0efa266-98a7-42da-8740-f3a7f7ffa3d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_253f58cc-2987-40cc-9d56-6dbef1ff049a" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_c0efa266-98a7-42da-8740-f3a7f7ffa3d9" xlink:to="loc_srt_RestatementDomain_253f58cc-2987-40cc-9d56-6dbef1ff049a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_ee41e583-0754-4c78-a050-89d3e397abb7" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScenarioPreviouslyReportedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_253f58cc-2987-40cc-9d56-6dbef1ff049a" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_ee41e583-0754-4c78-a050-89d3e397abb7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_a939dec4-17e8-472f-b22f-e55a228b5e25" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RevisionOfPriorPeriodReclassificationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_253f58cc-2987-40cc-9d56-6dbef1ff049a" xlink:to="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_a939dec4-17e8-472f-b22f-e55a228b5e25" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/OtherComprehensiveLossIncomeScheduleofComponentsofAccumulatedOtherComprehensiveLossIncomeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#OtherComprehensiveLossIncomeScheduleofComponentsofAccumulatedOtherComprehensiveLossIncomeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/OtherComprehensiveLossIncomeScheduleofComponentsofAccumulatedOtherComprehensiveLossIncomeDetails" xlink:type="extended" id="i86011a34c0f04aa7a5e1e974bff1d8bd_OtherComprehensiveLossIncomeScheduleofComponentsofAccumulatedOtherComprehensiveLossIncomeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_ddb67bec-2b22-4e8b-8b97-21154b4a6e11" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_56e0485e-3ef2-4ae8-b88a-a79317587e9d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AOCIAttributableToParentNetOfTaxRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_ddb67bec-2b22-4e8b-8b97-21154b4a6e11" xlink:to="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_56e0485e-3ef2-4ae8-b88a-a79317587e9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_d23d7ad7-598b-4180-b8d0-3c7facce3a18" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_56e0485e-3ef2-4ae8-b88a-a79317587e9d" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_d23d7ad7-598b-4180-b8d0-3c7facce3a18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_b2bc2556-41f5-4e49-8fce-5f922a8735b0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_56e0485e-3ef2-4ae8-b88a-a79317587e9d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_b2bc2556-41f5-4e49-8fce-5f922a8735b0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_0276e8a1-9a65-449f-a0bb-45fb0b508b9c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_56e0485e-3ef2-4ae8-b88a-a79317587e9d" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_0276e8a1-9a65-449f-a0bb-45fb0b508b9c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_f4b752af-9377-46ea-a1da-e3a9e491517d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_56e0485e-3ef2-4ae8-b88a-a79317587e9d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_f4b752af-9377-46ea-a1da-e3a9e491517d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_94d96ec4-881a-4b2d-a80c-b073855e4e20" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsLiabilitiesNet_ce7d2e09-b5f2-4c4a-8c98-6cdf183e5e65" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_ddb67bec-2b22-4e8b-8b97-21154b4a6e11" xlink:to="loc_us-gaap_DeferredTaxAssetsLiabilitiesNet_ce7d2e09-b5f2-4c4a-8c98-6cdf183e5e65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilities_3e5da26d-3484-4fdf-8bc0-e6b5144f4bf4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_ddb67bec-2b22-4e8b-8b97-21154b4a6e11" xlink:to="loc_us-gaap_DeferredTaxLiabilities_3e5da26d-3484-4fdf-8bc0-e6b5144f4bf4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_c2e8c710-a0a1-44d0-84fc-6495330edf48" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_ddb67bec-2b22-4e8b-8b97-21154b4a6e11" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_c2e8c710-a0a1-44d0-84fc-6495330edf48" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_87a5bd2b-f83d-4c0f-a475-5f5555cc0b7b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_c2e8c710-a0a1-44d0-84fc-6495330edf48" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_87a5bd2b-f83d-4c0f-a475-5f5555cc0b7b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_87a5bd2b-f83d-4c0f-a475-5f5555cc0b7b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_87a5bd2b-f83d-4c0f-a475-5f5555cc0b7b" xlink:to="loc_us-gaap_EquityComponentDomain_87a5bd2b-f83d-4c0f-a475-5f5555cc0b7b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_0dcaf4be-9358-4935-9b99-6486d42b474a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_87a5bd2b-f83d-4c0f-a475-5f5555cc0b7b" xlink:to="loc_us-gaap_EquityComponentDomain_0dcaf4be-9358-4935-9b99-6486d42b474a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_a39fafd5-97d0-427d-a811-4132e368c74b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_0dcaf4be-9358-4935-9b99-6486d42b474a" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_a39fafd5-97d0-427d-a811-4132e368c74b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_d9d5868f-05c4-4200-ba10-26d82c329a4b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_a39fafd5-97d0-427d-a811-4132e368c74b" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_d9d5868f-05c4-4200-ba10-26d82c329a4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_00edf6be-c6e2-44b5-ac3d-9770bd8e11b4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_a39fafd5-97d0-427d-a811-4132e368c74b" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_00edf6be-c6e2-44b5-ac3d-9770bd8e11b4" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/OtherComprehensiveLossIncomeScheduleOfComponentsOfComprehensiveLossIncomeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#OtherComprehensiveLossIncomeScheduleOfComponentsOfComprehensiveLossIncomeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/OtherComprehensiveLossIncomeScheduleOfComponentsOfComprehensiveLossIncomeDetails" xlink:type="extended" id="ic11bd7f0ff6a4df2bb9293675860d221_OtherComprehensiveLossIncomeScheduleOfComponentsOfComprehensiveLossIncomeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_991f26be-2b3b-42a3-a29e-efda22418d23" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_991f26be-2b3b-42a3-a29e-efda22418d23" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_434f7805-2ba5-4eb1-a797-622092efda44" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_434f7805-2ba5-4eb1-a797-622092efda44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_1826b6f1-78ec-442b-baa7-6f14d73e7d5f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_1826b6f1-78ec-442b-baa7-6f14d73e7d5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax_fb7e9000-4cc0-40e2-a0c8-65afee88bb8e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax_fb7e9000-4cc0-40e2-a0c8-65afee88bb8e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAociCurrentPeriodTax_0f88f685-e98c-403d-91fc-23e7379c92a0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationFromAociCurrentPeriodTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_ReclassificationFromAociCurrentPeriodTax_0f88f685-e98c-403d-91fc-23e7379c92a0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_40d28e50-6e2f-43a6-9bf8-a7f38e067109" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_40d28e50-6e2f-43a6-9bf8-a7f38e067109" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_35f93671-36ca-4296-8397-441e034edadf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_35f93671-36ca-4296-8397-441e034edadf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTax_5a5ac363-6249-4641-bafa-3becac7ac02c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTax_5a5ac363-6249-4641-bafa-3becac7ac02c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_4d388b03-5bec-4fc9-b251-af6734e5c267" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_4d388b03-5bec-4fc9-b251-af6734e5c267" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_57dac7a0-35a1-4f94-ae7b-6b9c2afd49e7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_abb7edc7-d636-47e2-82e2-b35f14ffba27" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_57dac7a0-35a1-4f94-ae7b-6b9c2afd49e7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_97fd899e-4a50-4ff7-b0ce-9061359beb05" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_57dac7a0-35a1-4f94-ae7b-6b9c2afd49e7" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_97fd899e-4a50-4ff7-b0ce-9061359beb05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_97fd899e-4a50-4ff7-b0ce-9061359beb05_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_97fd899e-4a50-4ff7-b0ce-9061359beb05" xlink:to="loc_us-gaap_EquityComponentDomain_97fd899e-4a50-4ff7-b0ce-9061359beb05_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6527f155-37a5-4be7-9149-59d4d00f4b9b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_97fd899e-4a50-4ff7-b0ce-9061359beb05" xlink:to="loc_us-gaap_EquityComponentDomain_6527f155-37a5-4be7-9149-59d4d00f4b9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_2bcd3b46-6438-4938-9eaf-8007d3bbd477" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedTranslationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6527f155-37a5-4be7-9149-59d4d00f4b9b" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_2bcd3b46-6438-4938-9eaf-8007d3bbd477" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember_22a8c67b-c30f-49f7-90cd-64a95b16862e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6527f155-37a5-4be7-9149-59d4d00f4b9b" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember_22a8c67b-c30f-49f7-90cd-64a95b16862e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember_76ac27bd-22f9-475e-8814-dd7f4440eae3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6527f155-37a5-4be7-9149-59d4d00f4b9b" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember_76ac27bd-22f9-475e-8814-dd7f4440eae3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8ab4ba36-e072-43f1-980b-09136ab29e66" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6527f155-37a5-4be7-9149-59d4d00f4b9b" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8ab4ba36-e072-43f1-980b-09136ab29e66" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentForeignCurrencyTranslationAdjustmentsAttributableToParentMember_1198c436-e251-4e05-b6b7-f15abb410055" xlink:href="mmc-20230630.xsd#mmc_AccumulatedDefinedBenefitPlansAdjustmentForeignCurrencyTranslationAdjustmentsAttributableToParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8ab4ba36-e072-43f1-980b-09136ab29e66" xlink:to="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentForeignCurrencyTranslationAdjustmentsAttributableToParentMember_1198c436-e251-4e05-b6b7-f15abb410055" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentRemeasurementAttributableToParentMember_f20e54de-d453-45ab-867e-b2a0f80117c3" xlink:href="mmc-20230630.xsd#mmc_AccumulatedDefinedBenefitPlansAdjustmentRemeasurementAttributableToParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8ab4ba36-e072-43f1-980b-09136ab29e66" xlink:to="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentRemeasurementAttributableToParentMember_f20e54de-d453-45ab-867e-b2a0f80117c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentSettlementAttributableToParentMember_99f6cdb7-f223-4a24-a961-1a3f40a65766" xlink:href="mmc-20230630.xsd#mmc_AccumulatedDefinedBenefitPlansAdjustmentSettlementAttributableToParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8ab4ba36-e072-43f1-980b-09136ab29e66" xlink:to="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentSettlementAttributableToParentMember_99f6cdb7-f223-4a24-a961-1a3f40a65766" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentOtherAdjustmentAttributableToParentMember_c3f4f29c-2595-429c-a6c2-158e891365df" xlink:href="mmc-20230630.xsd#mmc_AccumulatedDefinedBenefitPlansAdjustmentOtherAdjustmentAttributableToParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_8ab4ba36-e072-43f1-980b-09136ab29e66" xlink:to="loc_mmc_AccumulatedDefinedBenefitPlansAdjustmentOtherAdjustmentAttributableToParentMember_c3f4f29c-2595-429c-a6c2-158e891365df" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/AcquisitionsandDispositionsNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#AcquisitionsandDispositionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/AcquisitionsandDispositionsNarrativeDetails" xlink:type="extended" id="i4a055f6f732b4f668ad54bc17d25aee0_AcquisitionsandDispositionsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfBusinessesAcquired_4cfa18d9-d17a-430e-921d-020351802f1a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NumberOfBusinessesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_NumberOfBusinessesAcquired_4cfa18d9-d17a-430e-921d-020351802f1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_153d59e6-a1e4-40cb-bb80-dbb7ff747778" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_153d59e6-a1e4-40cb-bb80-dbb7ff747778" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_606e9e67-f94f-4343-bf93-e84f86176038" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_606e9e67-f94f-4343-bf93-e84f86176038" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationContingentandDeferredConsideration_e79b26bd-283e-4e17-9c2b-f4913b2552c5" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationContingentandDeferredConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_mmc_BusinessCombinationContingentandDeferredConsideration_e79b26bd-283e-4e17-9c2b-f4913b2552c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationContingentConsiderationArrangementsRevenueTargetPeriod_6ed81d6b-5f06-415b-ad52-6efca3bb5190" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationContingentConsiderationArrangementsRevenueTargetPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_mmc_BusinessCombinationContingentConsiderationArrangementsRevenueTargetPeriod_6ed81d6b-5f06-415b-ad52-6efca3bb5190" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_PaymentsForPreviousAcquisitionDeferredConsideration_37fe970e-35af-4f6c-a8b3-e5d2d911e6a3" xlink:href="mmc-20230630.xsd#mmc_PaymentsForPreviousAcquisitionDeferredConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_mmc_PaymentsForPreviousAcquisitionDeferredConsideration_37fe970e-35af-4f6c-a8b3-e5d2d911e6a3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_PaymentsForPreviousAcquisitionContingentConsideration_4e80cb47-f6fb-4bfb-8ce1-9082a844fbe0" xlink:href="mmc-20230630.xsd#mmc_PaymentsForPreviousAcquisitionContingentConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_mmc_PaymentsForPreviousAcquisitionContingentConsideration_4e80cb47-f6fb-4bfb-8ce1-9082a844fbe0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_e951fc1c-5b7c-4d31-ac77-498a89faf8a4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_e951fc1c-5b7c-4d31-ac77-498a89faf8a4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_0d701e94-e03c-4c12-9255-a609d4b46f88" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_0d701e94-e03c-4c12-9255-a609d4b46f88" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_d1c32e80-b6a6-4533-abea-3c438ed6ef02" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_d1c32e80-b6a6-4533-abea-3c438ed6ef02" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_d34b88d8-332a-42d0-9b3d-c1f0294eb24a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_d34b88d8-332a-42d0-9b3d-c1f0294eb24a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_e5d26d44-eee4-4da6-b2cc-f96a6d5ad043" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested_e5d26d44-eee4-4da6-b2cc-f96a6d5ad043" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_33f1843b-8b70-4919-a8b9-e6dcacd92d3b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_33f1843b-8b70-4919-a8b9-e6dcacd92d3b" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_4d134c46-4f25-4f41-a802-f9c6532d494f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProceedsFromDivestitureOfBusinesses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_ProceedsFromDivestitureOfBusinesses_4d134c46-4f25-4f41-a802-f9c6532d494f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal_5b260ad9-e4e8-4713-9728-3d42c32b7228" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal_5b260ad9-e4e8-4713-9728-3d42c32b7228" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeconsolidationGainOrLossAmount_e0fff9b5-1eae-43ec-8503-9c5f8fe3b88c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DeconsolidationGainOrLossAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_DeconsolidationGainOrLossAmount_e0fff9b5-1eae-43ec-8503-9c5f8fe3b88c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToMinorityShareholders_fcf5c95f-da24-4e29-99d8-57198e007469" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PaymentsToMinorityShareholders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_PaymentsToMinorityShareholders_fcf5c95f-da24-4e29-99d8-57198e007469" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3e1df262-aaee-4a1e-9add-599b6f499d4c" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_79c7160e-dba9-4986-b8e6-a9743e11fe4f" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:to="loc_srt_RangeAxis_79c7160e-dba9-4986-b8e6-a9743e11fe4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_79c7160e-dba9-4986-b8e6-a9743e11fe4f_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_79c7160e-dba9-4986-b8e6-a9743e11fe4f" xlink:to="loc_srt_RangeMember_79c7160e-dba9-4986-b8e6-a9743e11fe4f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_6e7a53ef-26ed-4727-9249-a3acf75a5f9a" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_79c7160e-dba9-4986-b8e6-a9743e11fe4f" xlink:to="loc_srt_RangeMember_6e7a53ef-26ed-4727-9249-a3acf75a5f9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_333e2800-161c-42c2-aff5-3d6f527689b4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_6e7a53ef-26ed-4727-9249-a3acf75a5f9a" xlink:to="loc_srt_MinimumMember_333e2800-161c-42c2-aff5-3d6f527689b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_67485e4f-8ebd-46fb-a613-f4eb4cde316e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_6e7a53ef-26ed-4727-9249-a3acf75a5f9a" xlink:to="loc_srt_MaximumMember_67485e4f-8ebd-46fb-a613-f4eb4cde316e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_dab991fa-8448-40fd-9be1-361ce36a62f7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_dab991fa-8448-40fd-9be1-361ce36a62f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_dab991fa-8448-40fd-9be1-361ce36a62f7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_dab991fa-8448-40fd-9be1-361ce36a62f7" xlink:to="loc_us-gaap_SegmentDomain_dab991fa-8448-40fd-9be1-361ce36a62f7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_aa003afe-c974-44c5-a07c-cdddaf2f1f5f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_dab991fa-8448-40fd-9be1-361ce36a62f7" xlink:to="loc_us-gaap_SegmentDomain_aa003afe-c974-44c5-a07c-cdddaf2f1f5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SmallIndividualFinancialAdvisoryBusinessMember_b9dbbbfc-94eb-4772-afb1-a9b311b9b3fa" xlink:href="mmc-20230630.xsd#mmc_SmallIndividualFinancialAdvisoryBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_aa003afe-c974-44c5-a07c-cdddaf2f1f5f" xlink:to="loc_mmc_SmallIndividualFinancialAdvisoryBusinessMember_b9dbbbfc-94eb-4772-afb1-a9b311b9b3fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RiskandInsuranceServicesSegmentMember_cd9e0694-e407-4be5-8384-0f0d38c44b6f" xlink:href="mmc-20230630.xsd#mmc_RiskandInsuranceServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_aa003afe-c974-44c5-a07c-cdddaf2f1f5f" xlink:to="loc_mmc_RiskandInsuranceServicesSegmentMember_cd9e0694-e407-4be5-8384-0f0d38c44b6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ConsultingSegmentMember_5b4f24ae-05be-4e4e-9b20-90f5d965e6ac" xlink:href="mmc-20230630.xsd#mmc_ConsultingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_aa003afe-c974-44c5-a07c-cdddaf2f1f5f" xlink:to="loc_mmc_ConsultingSegmentMember_5b4f24ae-05be-4e4e-9b20-90f5d965e6ac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_594673a2-4fec-446f-9745-f5bdf73644c3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_594673a2-4fec-446f-9745-f5bdf73644c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_594673a2-4fec-446f-9745-f5bdf73644c3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_594673a2-4fec-446f-9745-f5bdf73644c3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_594673a2-4fec-446f-9745-f5bdf73644c3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_89ceecf7-862e-4e14-ae6b-0e7e58946116" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_594673a2-4fec-446f-9745-f5bdf73644c3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_89ceecf7-862e-4e14-ae6b-0e7e58946116" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_PriorFiscalPeriodsAcquisitionsMember_8aed537a-d848-4eff-8c10-ee0af0971393" xlink:href="mmc-20230630.xsd#mmc_PriorFiscalPeriodsAcquisitionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_89ceecf7-862e-4e14-ae6b-0e7e58946116" xlink:to="loc_mmc_PriorFiscalPeriodsAcquisitionsMember_8aed537a-d848-4eff-8c10-ee0af0971393" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_AdvanceAssetManagementMember_cf298bf7-2963-4586-9a60-fdc5dc5faf73" xlink:href="mmc-20230630.xsd#mmc_AdvanceAssetManagementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_89ceecf7-862e-4e14-ae6b-0e7e58946116" xlink:to="loc_mmc_AdvanceAssetManagementMember_cf298bf7-2963-4586-9a60-fdc5dc5faf73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_CurrentFiscalPeriodAcquisitionsMember_adf4ad79-c439-4799-8712-9fc09977d807" xlink:href="mmc-20230630.xsd#mmc_CurrentFiscalPeriodAcquisitionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_89ceecf7-862e-4e14-ae6b-0e7e58946116" xlink:to="loc_mmc_CurrentFiscalPeriodAcquisitionsMember_adf4ad79-c439-4799-8712-9fc09977d807" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_cf79f195-ebb2-4232-acfa-5e8e1f08f8c1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_cf79f195-ebb2-4232-acfa-5e8e1f08f8c1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_cf79f195-ebb2-4232-acfa-5e8e1f08f8c1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_cf79f195-ebb2-4232-acfa-5e8e1f08f8c1" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_cf79f195-ebb2-4232-acfa-5e8e1f08f8c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_11322b57-5341-4f30-be01-07a35c151180" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_cf79f195-ebb2-4232-acfa-5e8e1f08f8c1" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_11322b57-5341-4f30-be01-07a35c151180" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_USAffinityBusinessMember_40787b7f-a3e6-4c7f-aa5d-8bc854e781ac" xlink:href="mmc-20230630.xsd#mmc_USAffinityBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_11322b57-5341-4f30-be01-07a35c151180" xlink:to="loc_mmc_USAffinityBusinessMember_40787b7f-a3e6-4c7f-aa5d-8bc854e781ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RetirementPlanAdministrationAndCallCenterOperationsInBrazilMember_26af0922-9e1e-42ea-9472-f61b4ccbb96f" xlink:href="mmc-20230630.xsd#mmc_RetirementPlanAdministrationAndCallCenterOperationsInBrazilMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_11322b57-5341-4f30-be01-07a35c151180" xlink:to="loc_mmc_RetirementPlanAdministrationAndCallCenterOperationsInBrazilMember_26af0922-9e1e-42ea-9472-f61b4ccbb96f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_4469f2fd-1544-4293-bbb6-07fb22aa91d4" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:to="loc_srt_ConsolidationItemsAxis_4469f2fd-1544-4293-bbb6-07fb22aa91d4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_4469f2fd-1544-4293-bbb6-07fb22aa91d4_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_4469f2fd-1544-4293-bbb6-07fb22aa91d4" xlink:to="loc_srt_ConsolidationItemsDomain_4469f2fd-1544-4293-bbb6-07fb22aa91d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_f9b81cdb-a0d3-4227-b1a6-5389f06e0ddf" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_4469f2fd-1544-4293-bbb6-07fb22aa91d4" xlink:to="loc_srt_ConsolidationItemsDomain_f9b81cdb-a0d3-4227-b1a6-5389f06e0ddf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_c7bc395f-d048-46e4-90d7-f6ae776b7058" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_f9b81cdb-a0d3-4227-b1a6-5389f06e0ddf" xlink:to="loc_us-gaap_OperatingSegmentsMember_c7bc395f-d048-46e4-90d7-f6ae776b7058" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8e387f6e-50b3-4e54-bf3c-ac17f3535386" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_efbac418-f860-4d01-8617-fb497d42d87d" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8e387f6e-50b3-4e54-bf3c-ac17f3535386" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_8e387f6e-50b3-4e54-bf3c-ac17f3535386_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8e387f6e-50b3-4e54-bf3c-ac17f3535386" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_8e387f6e-50b3-4e54-bf3c-ac17f3535386_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_80981fb2-a0cc-4e45-a241-6c8942f3bd47" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8e387f6e-50b3-4e54-bf3c-ac17f3535386" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_80981fb2-a0cc-4e45-a241-6c8942f3bd47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BeassurSARLMember_f077f8d8-963a-4637-8726-0112f93d8a03" xlink:href="mmc-20230630.xsd#mmc_BeassurSARLMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_80981fb2-a0cc-4e45-a241-6c8942f3bd47" xlink:to="loc_mmc_BeassurSARLMember_f077f8d8-963a-4637-8726-0112f93d8a03" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/AcquisitionsandDispositionsAllocationOfAcquisitionCostsDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#AcquisitionsandDispositionsAllocationOfAcquisitionCostsDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/AcquisitionsandDispositionsAllocationOfAcquisitionCostsDetails" xlink:type="extended" id="i5cd9e5baa5eb45aea4be6956fb7735c5_AcquisitionsandDispositionsAllocationOfAcquisitionCostsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_61331d76-be28-48d5-ba9a-c59e8a5fe247" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_9f14c278-5804-4a59-8b50-dfc65ce59a87" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_61331d76-be28-48d5-ba9a-c59e8a5fe247" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_9f14c278-5804-4a59-8b50-dfc65ce59a87" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationContingentandDeferredConsideration_0643d986-1cff-48b4-b6ca-1ddb4d5a629a" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationContingentandDeferredConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_61331d76-be28-48d5-ba9a-c59e8a5fe247" xlink:to="loc_mmc_BusinessCombinationContingentandDeferredConsideration_0643d986-1cff-48b4-b6ca-1ddb4d5a629a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_bd2b624e-fa26-41db-ab38-09fd572f68c7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_61331d76-be28-48d5-ba9a-c59e8a5fe247" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_bd2b624e-fa26-41db-ab38-09fd572f68c7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_61331d76-be28-48d5-ba9a-c59e8a5fe247" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_776a02ff-e2b2-4d62-880e-efe8ac03676d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_776a02ff-e2b2-4d62-880e-efe8ac03676d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalentsHeldInAFiduciaryCapacity_5974337a-ae77-43d7-aa7b-f5d8008ec3b1" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalentsHeldInAFiduciaryCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalentsHeldInAFiduciaryCapacity_5974337a-ae77-43d7-aa7b-f5d8008ec3b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_bd4e8d59-4cb3-47db-a59d-bf1d7aed132b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_bd4e8d59-4cb3-47db-a59d-bf1d7aed132b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_1227f0b8-e1d6-4f22-9435-367cc1c303bb" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_Goodwill_1227f0b8-e1d6-4f22-9435-367cc1c303bb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_b8c075b4-fa2d-431b-841b-3343a36d3630" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_b8c075b4-fa2d-431b-841b-3343a36d3630" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAssetsIncludingGoodwill_126d2cf7-3acf-4cef-8efe-1555dca6f585" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAssetsIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAssetsIncludingGoodwill_126d2cf7-3acf-4cef-8efe-1555dca6f585" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_4dceb40c-71c8-43d9-ac6d-d6fd45eee03a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_4dceb40c-71c8-43d9-ac6d-d6fd45eee03a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFiduciaryLiabilities_ec42a5d8-f68a-43c4-aac6-8c1ff7b3fb98" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFiduciaryLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_mmc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFiduciaryLiabilities_ec42a5d8-f68a-43c4-aac6-8c1ff7b3fb98" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_2268e320-5de1-416a-bdf9-06e925def2f3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_2268e320-5de1-416a-bdf9-06e925def2f3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_d480720d-0d78-4fcf-ab74-b0e6d9057151" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_d480720d-0d78-4fcf-ab74-b0e6d9057151" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedLessNoncontrollingInterest_56471fb3-2c19-4b6d-922a-1f496434978a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedLessNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_fd6d4175-c3d6-4ede-ba88-a87e2f588fe3" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedLessNoncontrollingInterest_56471fb3-2c19-4b6d-922a-1f496434978a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5c023640-2033-44a3-9c6f-72dd73aeb3f5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_61331d76-be28-48d5-ba9a-c59e8a5fe247" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5c023640-2033-44a3-9c6f-72dd73aeb3f5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_e38160aa-841d-401b-97b3-29b5d3a1bb86" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5c023640-2033-44a3-9c6f-72dd73aeb3f5" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_e38160aa-841d-401b-97b3-29b5d3a1bb86" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e38160aa-841d-401b-97b3-29b5d3a1bb86_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e38160aa-841d-401b-97b3-29b5d3a1bb86" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e38160aa-841d-401b-97b3-29b5d3a1bb86_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2d5db9f1-5df7-4dcd-8356-9abec5de6f03" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_e38160aa-841d-401b-97b3-29b5d3a1bb86" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2d5db9f1-5df7-4dcd-8356-9abec5de6f03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_CurrentFiscalPeriodAcquisitionsMember_9adf05b5-227a-421c-bb6b-6e646f2b9cae" xlink:href="mmc-20230630.xsd#mmc_CurrentFiscalPeriodAcquisitionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2d5db9f1-5df7-4dcd-8356-9abec5de6f03" xlink:to="loc_mmc_CurrentFiscalPeriodAcquisitionsMember_9adf05b5-227a-421c-bb6b-6e646f2b9cae" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/AcquisitionsandDispositionsAcquiredFiniteLivedIntangibleAssetsDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#AcquisitionsandDispositionsAcquiredFiniteLivedIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/AcquisitionsandDispositionsAcquiredFiniteLivedIntangibleAssetsDetails" xlink:type="extended" id="i9de5580a912e4ac786571e4efc159e4e_AcquisitionsandDispositionsAcquiredFiniteLivedIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_f0ce1f6b-b299-4a32-862f-6da7d59bb923" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_813d9daa-2e73-48e7-8fec-c1c609a9b703" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_f0ce1f6b-b299-4a32-862f-6da7d59bb923" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_813d9daa-2e73-48e7-8fec-c1c609a9b703" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_8b70dbea-45f2-4c53-8a04-c2a172442d31" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_f0ce1f6b-b299-4a32-862f-6da7d59bb923" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_8b70dbea-45f2-4c53-8a04-c2a172442d31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_89e1e4a0-3b08-416d-8bd3-2adda9e6ab26" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_f0ce1f6b-b299-4a32-862f-6da7d59bb923" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_89e1e4a0-3b08-416d-8bd3-2adda9e6ab26" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_37981720-fcf4-4eb7-a46b-f72dbb4adade" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_89e1e4a0-3b08-416d-8bd3-2adda9e6ab26" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_37981720-fcf4-4eb7-a46b-f72dbb4adade" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_37981720-fcf4-4eb7-a46b-f72dbb4adade_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_37981720-fcf4-4eb7-a46b-f72dbb4adade" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_37981720-fcf4-4eb7-a46b-f72dbb4adade_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_83225211-c2b5-4538-9e12-39fa88a86d3b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_37981720-fcf4-4eb7-a46b-f72dbb4adade" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_83225211-c2b5-4538-9e12-39fa88a86d3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_91a0b9b8-92a9-4c70-88a6-e4c26583c9ab" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_83225211-c2b5-4538-9e12-39fa88a86d3b" xlink:to="loc_us-gaap_CustomerRelationshipsMember_91a0b9b8-92a9-4c70-88a6-e4c26583c9ab" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/GoodwillandOtherIntangiblesNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#GoodwillandOtherIntangiblesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/GoodwillandOtherIntangiblesNarrativeDetails" xlink:type="extended" id="i69049932b10b4bb78b486bf6fd186112_GoodwillandOtherIntangiblesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_bacea17a-029d-4b59-ac81-b222f2ef62b6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_f1caa2ee-c198-407c-9a30-d6dac9c111d0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_bacea17a-029d-4b59-ac81-b222f2ef62b6" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_f1caa2ee-c198-407c-9a30-d6dac9c111d0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_3058e0fe-74cd-413a-9f2d-078937cf42fc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_bacea17a-029d-4b59-ac81-b222f2ef62b6" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_3058e0fe-74cd-413a-9f2d-078937cf42fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_ff878cd7-e94b-4e35-9f37-52c106bfb4c8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_bacea17a-029d-4b59-ac81-b222f2ef62b6" xlink:to="loc_us-gaap_Goodwill_ff878cd7-e94b-4e35-9f37-52c106bfb4c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_92025736-e083-4d64-97b1-c0522f09bfa1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_bacea17a-029d-4b59-ac81-b222f2ef62b6" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_92025736-e083-4d64-97b1-c0522f09bfa1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_a7baf3aa-04d5-4c70-ba33-f66da1e1c64f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_bacea17a-029d-4b59-ac81-b222f2ef62b6" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_a7baf3aa-04d5-4c70-ba33-f66da1e1c64f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_fbd9e2d0-d06e-4dff-9fed-55b19a1c0472" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_a7baf3aa-04d5-4c70-ba33-f66da1e1c64f" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_fbd9e2d0-d06e-4dff-9fed-55b19a1c0472" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_fbd9e2d0-d06e-4dff-9fed-55b19a1c0472_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_fbd9e2d0-d06e-4dff-9fed-55b19a1c0472" xlink:to="loc_us-gaap_SegmentDomain_fbd9e2d0-d06e-4dff-9fed-55b19a1c0472_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_5b9b46fa-0073-4b9f-9211-347b3ad14dd2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_fbd9e2d0-d06e-4dff-9fed-55b19a1c0472" xlink:to="loc_us-gaap_SegmentDomain_5b9b46fa-0073-4b9f-9211-347b3ad14dd2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RiskandInsuranceServicesSegmentMember_fcb39883-c719-4375-b911-bdce1f7538a6" xlink:href="mmc-20230630.xsd#mmc_RiskandInsuranceServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_5b9b46fa-0073-4b9f-9211-347b3ad14dd2" xlink:to="loc_mmc_RiskandInsuranceServicesSegmentMember_fcb39883-c719-4375-b911-bdce1f7538a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ConsultingSegmentMember_42a636c5-b898-4231-952e-8053f852c9f2" xlink:href="mmc-20230630.xsd#mmc_ConsultingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_5b9b46fa-0073-4b9f-9211-347b3ad14dd2" xlink:to="loc_mmc_ConsultingSegmentMember_42a636c5-b898-4231-952e-8053f852c9f2" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/GoodwillandOtherIntangiblesAmortizedIntangibleAssetsDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#GoodwillandOtherIntangiblesAmortizedIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/GoodwillandOtherIntangiblesAmortizedIntangibleAssetsDetails" xlink:type="extended" id="icb278d1e881d45ac903111cd72719f13_GoodwillandOtherIntangiblesAmortizedIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00783f58-5425-49fd-a9ba-ad39176b53a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_38810ebf-400f-42f3-a4f5-bccd4932b27c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00783f58-5425-49fd-a9ba-ad39176b53a6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_38810ebf-400f-42f3-a4f5-bccd4932b27c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_47da8626-0ece-45af-bc4e-c5da55e9184e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00783f58-5425-49fd-a9ba-ad39176b53a6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_47da8626-0ece-45af-bc4e-c5da55e9184e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_89e47e6e-41f4-44de-a026-596a813244db" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00783f58-5425-49fd-a9ba-ad39176b53a6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_89e47e6e-41f4-44de-a026-596a813244db" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_d65e1be1-42dd-4bf8-8855-152b59c514d3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00783f58-5425-49fd-a9ba-ad39176b53a6" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_d65e1be1-42dd-4bf8-8855-152b59c514d3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_dc69706a-e650-4be3-9300-13917dbc9c65" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_d65e1be1-42dd-4bf8-8855-152b59c514d3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_dc69706a-e650-4be3-9300-13917dbc9c65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_dc69706a-e650-4be3-9300-13917dbc9c65_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_dc69706a-e650-4be3-9300-13917dbc9c65" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_dc69706a-e650-4be3-9300-13917dbc9c65_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_c9f50e92-4e22-4aad-9866-1091d1fc4b92" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_dc69706a-e650-4be3-9300-13917dbc9c65" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_c9f50e92-4e22-4aad-9866-1091d1fc4b92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_2087c932-f9bd-4804-8a34-67b3ed4f1bde" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_c9f50e92-4e22-4aad-9866-1091d1fc4b92" xlink:to="loc_us-gaap_CustomerRelationshipsMember_2087c932-f9bd-4804-8a34-67b3ed4f1bde" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_185381a9-ad07-4397-8f55-f900445ad9f3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_c9f50e92-4e22-4aad-9866-1091d1fc4b92" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_185381a9-ad07-4397-8f55-f900445ad9f3" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#FairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended" id="i5f22ad27f216467f97cf2b5434add40b_FairValueMeasurementsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationContingentConsiderationArrangementsRevenueTargetPeriod_4af879e5-7f54-4551-89aa-647260188ffa" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationContingentConsiderationArrangementsRevenueTargetPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_mmc_BusinessCombinationContingentConsiderationArrangementsRevenueTargetPeriod_4af879e5-7f54-4551-89aa-647260188ffa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ProceedsFromContingentPurchaseConsiderationAsset_d1855736-3226-444d-a72a-ba96808a0ca3" xlink:href="mmc-20230630.xsd#mmc_ProceedsFromContingentPurchaseConsiderationAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_mmc_ProceedsFromContingentPurchaseConsiderationAsset_d1855736-3226-444d-a72a-ba96808a0ca3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestments_1a1a5250-cafc-460c-93e9-0c6c037a5ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_us-gaap_EquityMethodInvestments_1a1a5250-cafc-460c-93e9-0c6c037a5ce5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_5bd1acc4-30fa-413e-bf3b-df138945b13c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_5bd1acc4-30fa-413e-bf3b-df138945b13c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_CommitmentsForPotentialFutureInvestments_d62800ce-a63d-4d63-a685-00b4a3900e6a" xlink:href="mmc-20230630.xsd#mmc_CommitmentsForPotentialFutureInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_mmc_CommitmentsForPotentialFutureInvestments_d62800ce-a63d-4d63-a685-00b4a3900e6a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNi_aec7187f-bad5-4a9e-a361-c596903b0047" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquitySecuritiesFvNi"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_us-gaap_EquitySecuritiesFvNi_aec7187f-bad5-4a9e-a361-c596903b0047" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiGainLoss_4179d377-fccc-4db7-a54f-6b1cb441cd02" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquitySecuritiesFvNiGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_us-gaap_EquitySecuritiesFvNiGainLoss_4179d377-fccc-4db7-a54f-6b1cb441cd02" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_79f7157f-e537-4d4e-b79c-ec81ac34f7c1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_79f7157f-e537-4d4e-b79c-ec81ac34f7c1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5ac75a4-9f2b-4492-aff4-645f8f4cad31" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_f31dc772-d7b8-4d3d-8451-9f9ee44300f1" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5ac75a4-9f2b-4492-aff4-645f8f4cad31" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_20776b00-c78c-4a99-85ae-9bb77e17dbf7" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5ac75a4-9f2b-4492-aff4-645f8f4cad31" xlink:to="loc_srt_RangeAxis_20776b00-c78c-4a99-85ae-9bb77e17dbf7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_20776b00-c78c-4a99-85ae-9bb77e17dbf7_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_20776b00-c78c-4a99-85ae-9bb77e17dbf7" xlink:to="loc_srt_RangeMember_20776b00-c78c-4a99-85ae-9bb77e17dbf7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_3a341b9e-23b3-40cb-805d-3af7ef74270e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_20776b00-c78c-4a99-85ae-9bb77e17dbf7" xlink:to="loc_srt_RangeMember_3a341b9e-23b3-40cb-805d-3af7ef74270e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_d9ae14cf-bcbf-4348-87c5-137530a0adaa" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_3a341b9e-23b3-40cb-805d-3af7ef74270e" xlink:to="loc_srt_MinimumMember_d9ae14cf-bcbf-4348-87c5-137530a0adaa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_041f4c68-8520-45ba-b588-a05d1d5e9ecf" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_3a341b9e-23b3-40cb-805d-3af7ef74270e" xlink:to="loc_srt_MaximumMember_041f4c68-8520-45ba-b588-a05d1d5e9ecf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d9cd6fe7-14cc-490e-903f-8bb8f7040ca2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5ac75a4-9f2b-4492-aff4-645f8f4cad31" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d9cd6fe7-14cc-490e-903f-8bb8f7040ca2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d9cd6fe7-14cc-490e-903f-8bb8f7040ca2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d9cd6fe7-14cc-490e-903f-8bb8f7040ca2" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d9cd6fe7-14cc-490e-903f-8bb8f7040ca2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_05c3d1e2-3299-4900-98b1-c824202351ab" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d9cd6fe7-14cc-490e-903f-8bb8f7040ca2" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_05c3d1e2-3299-4900-98b1-c824202351ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_71c65055-d271-4bd7-ad54-84fa448e6dc7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_05c3d1e2-3299-4900-98b1-c824202351ab" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_71c65055-d271-4bd7-ad54-84fa448e6dc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdd6ff00-a48f-485c-94bc-67b75398f1a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5ac75a4-9f2b-4492-aff4-645f8f4cad31" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdd6ff00-a48f-485c-94bc-67b75398f1a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_cdd6ff00-a48f-485c-94bc-67b75398f1a8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdd6ff00-a48f-485c-94bc-67b75398f1a8" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_cdd6ff00-a48f-485c-94bc-67b75398f1a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_9f032698-99fa-4948-8511-8fcadc140a0a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdd6ff00-a48f-485c-94bc-67b75398f1a8" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_9f032698-99fa-4948-8511-8fcadc140a0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_0c991d21-0f5c-42da-a2ef-7c44d9b5a6e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_9f032698-99fa-4948-8511-8fcadc140a0a" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_0c991d21-0f5c-42da-a2ef-7c44d9b5a6e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_fe9ba9ba-8f7f-4b72-bc4a-54ecd5a21c66" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_a5ac75a4-9f2b-4492-aff4-645f8f4cad31" xlink:to="loc_us-gaap_InvestmentTypeAxis_fe9ba9ba-8f7f-4b72-bc4a-54ecd5a21c66" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_fe9ba9ba-8f7f-4b72-bc4a-54ecd5a21c66_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_fe9ba9ba-8f7f-4b72-bc4a-54ecd5a21c66" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_fe9ba9ba-8f7f-4b72-bc4a-54ecd5a21c66_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_6765cb4e-7cb1-48a4-b2a8-d831de53e152" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_fe9ba9ba-8f7f-4b72-bc4a-54ecd5a21c66" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_6765cb4e-7cb1-48a4-b2a8-d831de53e152" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_PrivateInsuranceandConsultingMember_23caaeb8-4ee2-4e4a-85c9-937996bbcab5" xlink:href="mmc-20230630.xsd#mmc_PrivateInsuranceandConsultingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_6765cb4e-7cb1-48a4-b2a8-d831de53e152" xlink:to="loc_mmc_PrivateInsuranceandConsultingMember_23caaeb8-4ee2-4e4a-85c9-937996bbcab5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivateEquityFundsMember_a39bc045-691c-4a33-b968-08bb28e9cbcd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PrivateEquityFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_6765cb4e-7cb1-48a4-b2a8-d831de53e152" xlink:to="loc_us-gaap_PrivateEquityFundsMember_a39bc045-691c-4a33-b968-08bb28e9cbcd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_8b1fa53d-9f3f-4f36-a101-ec4cfdb3455e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_6765cb4e-7cb1-48a4-b2a8-d831de53e152" xlink:to="loc_us-gaap_EquitySecuritiesMember_8b1fa53d-9f3f-4f36-a101-ec4cfdb3455e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/FairValueMeasurementsAssetsAndLiabilitiesMeasuredAtFairValueOnARecurringBasisDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#FairValueMeasurementsAssetsAndLiabilitiesMeasuredAtFairValueOnARecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/FairValueMeasurementsAssetsAndLiabilitiesMeasuredAtFairValueOnARecurringBasisDetails" xlink:type="extended" id="if49412086a854d13a71ae99861bbdd2f_FairValueMeasurementsAssetsAndLiabilitiesMeasuredAtFairValueOnARecurringBasisDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_c32c1740-4ee7-4739-84b1-037c842a4efc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_c32c1740-4ee7-4739-84b1-037c842a4efc" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNi_eedb5f64-fb06-4262-b5e1-ad59044e124d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquitySecuritiesFvNi"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:to="loc_us-gaap_EquitySecuritiesFvNi_eedb5f64-fb06-4262-b5e1-ad59044e124d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsFairValueDisclosure_b2d9298d-45cb-4199-80f7-ab0d17a6ff33" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:to="loc_us-gaap_OtherAssetsFairValueDisclosure_b2d9298d-45cb-4199-80f7-ab0d17a6ff33" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_671f1fdb-a1cd-48d1-8891-4e3646740e75" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_671f1fdb-a1cd-48d1-8891-4e3646740e75" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateFairValueHedgeAssetAtFairValue_e81c873e-2476-41a9-96ef-36451ceae931" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_InterestRateFairValueHedgeAssetAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:to="loc_us-gaap_InterestRateFairValueHedgeAssetAtFairValue_e81c873e-2476-41a9-96ef-36451ceae931" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationContingentConsiderationAssetFairValueDisclosure_c94d9066-1a28-4897-9336-5adbcade982e" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationContingentConsiderationAssetFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:to="loc_mmc_BusinessCombinationContingentConsiderationAssetFairValueDisclosure_c94d9066-1a28-4897-9336-5adbcade982e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_bbc1dd7f-e9dc-4e14-a00c-e25c7626790f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_7698e66f-471d-4176-8f4b-5d564ca400c4" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_bbc1dd7f-e9dc-4e14-a00c-e25c7626790f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_FiduciaryAssetsAbstract_76341132-6af0-4d7d-b6d7-fbce64f1ac17" xlink:href="mmc-20230630.xsd#mmc_FiduciaryAssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_c32c1740-4ee7-4739-84b1-037c842a4efc" xlink:to="loc_mmc_FiduciaryAssetsAbstract_76341132-6af0-4d7d-b6d7-fbce64f1ac17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_FiduciaryAssetsFairValueDisclosure_27aafe54-023f-492a-bb8c-2695422e7d7f" xlink:href="mmc-20230630.xsd#mmc_FiduciaryAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mmc_FiduciaryAssetsAbstract_76341132-6af0-4d7d-b6d7-fbce64f1ac17" xlink:to="loc_mmc_FiduciaryAssetsFairValueDisclosure_27aafe54-023f-492a-bb8c-2695422e7d7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_b9f6017b-7e11-445b-92f4-820a3ce6e377" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_c32c1740-4ee7-4739-84b1-037c842a4efc" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_b9f6017b-7e11-445b-92f4-820a3ce6e377" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_BusinessCombinationContingentConsiderationLiabilityFairValueDisclosure_39f56694-a711-4000-977d-3f94f59302d2" xlink:href="mmc-20230630.xsd#mmc_BusinessCombinationContingentConsiderationLiabilityFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_b9f6017b-7e11-445b-92f4-820a3ce6e377" xlink:to="loc_mmc_BusinessCombinationContingentConsiderationLiabilityFairValueDisclosure_39f56694-a711-4000-977d-3f94f59302d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_38c6292c-e7a6-4ad9-af32-9658bf3f620c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_b9f6017b-7e11-445b-92f4-820a3ce6e377" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_38c6292c-e7a6-4ad9-af32-9658bf3f620c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_42b4620d-2c41-406c-9488-1724b7125f23" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_c32c1740-4ee7-4739-84b1-037c842a4efc" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_42b4620d-2c41-406c-9488-1724b7125f23" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_e6d1e7b4-657d-43de-8764-e6fa5327b615" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_42b4620d-2c41-406c-9488-1724b7125f23" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_e6d1e7b4-657d-43de-8764-e6fa5327b615" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_e6d1e7b4-657d-43de-8764-e6fa5327b615_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_e6d1e7b4-657d-43de-8764-e6fa5327b615" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_e6d1e7b4-657d-43de-8764-e6fa5327b615_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_56d362be-24ba-48ac-bdac-24a589f68ced" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_e6d1e7b4-657d-43de-8764-e6fa5327b615" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_56d362be-24ba-48ac-bdac-24a589f68ced" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsMember_d60ae3a9-6e17-4870-8d2d-cb34c7778dd5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_56d362be-24ba-48ac-bdac-24a589f68ced" xlink:to="loc_us-gaap_OtherAssetsMember_d60ae3a9-6e17-4870-8d2d-cb34c7778dd5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsMember_b4bce8e4-dc3a-4254-916a-314a97ee471e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_56d362be-24ba-48ac-bdac-24a589f68ced" xlink:to="loc_us-gaap_CashAndCashEquivalentsMember_b4bce8e4-dc3a-4254-916a-314a97ee471e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_OtherReceivablesMember_0cbc1192-bb25-496e-a9d7-2df32043cb1e" xlink:href="mmc-20230630.xsd#mmc_OtherReceivablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_56d362be-24ba-48ac-bdac-24a589f68ced" xlink:to="loc_mmc_OtherReceivablesMember_0cbc1192-bb25-496e-a9d7-2df32043cb1e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_AccountsPayableandAccruedLiabilitiesandOtherLiabilitiesMember_8a224dd8-d133-47b1-872c-dee8494cd2ee" xlink:href="mmc-20230630.xsd#mmc_AccountsPayableandAccruedLiabilitiesandOtherLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_56d362be-24ba-48ac-bdac-24a589f68ced" xlink:to="loc_mmc_AccountsPayableandAccruedLiabilitiesandOtherLiabilitiesMember_8a224dd8-d133-47b1-872c-dee8494cd2ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_693b7cbe-01f0-4d06-9b8c-e1f350306f64" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_42b4620d-2c41-406c-9488-1724b7125f23" xlink:to="loc_us-gaap_FinancialInstrumentAxis_693b7cbe-01f0-4d06-9b8c-e1f350306f64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_693b7cbe-01f0-4d06-9b8c-e1f350306f64_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_693b7cbe-01f0-4d06-9b8c-e1f350306f64" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_693b7cbe-01f0-4d06-9b8c-e1f350306f64_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_e883ba80-480b-4f0c-a37b-d6f5037e94b3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_693b7cbe-01f0-4d06-9b8c-e1f350306f64" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_e883ba80-480b-4f0c-a37b-d6f5037e94b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_95f6aac3-91b9-4b35-9795-a2b03c854191" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_e883ba80-480b-4f0c-a37b-d6f5037e94b3" xlink:to="loc_us-gaap_MoneyMarketFundsMember_95f6aac3-91b9-4b35-9795-a2b03c854191" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_fbc45f7b-62a8-484e-9cc6-06507114a437" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_42b4620d-2c41-406c-9488-1724b7125f23" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_fbc45f7b-62a8-484e-9cc6-06507114a437" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_fbc45f7b-62a8-484e-9cc6-06507114a437_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_fbc45f7b-62a8-484e-9cc6-06507114a437" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_fbc45f7b-62a8-484e-9cc6-06507114a437_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c26b15e7-71ad-48e3-989d-2bdc7a996df6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_fbc45f7b-62a8-484e-9cc6-06507114a437" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c26b15e7-71ad-48e3-989d-2bdc7a996df6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_7a147a9c-0bf9-4067-9014-a920e6e92387" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c26b15e7-71ad-48e3-989d-2bdc7a996df6" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_7a147a9c-0bf9-4067-9014-a920e6e92387" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_d9072c9c-a976-45d4-bc59-63b6fc1606b2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c26b15e7-71ad-48e3-989d-2bdc7a996df6" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_d9072c9c-a976-45d4-bc59-63b6fc1606b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_d67c696e-30bd-48cd-a48b-38e9a0d5b1c2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c26b15e7-71ad-48e3-989d-2bdc7a996df6" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_d67c696e-30bd-48cd-a48b-38e9a0d5b1c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4f862573-f757-4f09-b74b-1ca41eb8c88e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_42b4620d-2c41-406c-9488-1724b7125f23" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4f862573-f757-4f09-b74b-1ca41eb8c88e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_4f862573-f757-4f09-b74b-1ca41eb8c88e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4f862573-f757-4f09-b74b-1ca41eb8c88e" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_4f862573-f757-4f09-b74b-1ca41eb8c88e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_12360711-2feb-481a-b3c1-ddc1f5099143" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4f862573-f757-4f09-b74b-1ca41eb8c88e" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_12360711-2feb-481a-b3c1-ddc1f5099143" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_3f6a76a0-1478-450c-a6eb-7835763cb3ef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_12360711-2feb-481a-b3c1-ddc1f5099143" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_3f6a76a0-1478-450c-a6eb-7835763cb3ef" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/FairValueMeasurementsChangesInFairValueOfLevel3LiabilitiesRepresentingAcquisitionRelatedContingentConsiderationDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#FairValueMeasurementsChangesInFairValueOfLevel3LiabilitiesRepresentingAcquisitionRelatedContingentConsiderationDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/FairValueMeasurementsChangesInFairValueOfLevel3LiabilitiesRepresentingAcquisitionRelatedContingentConsiderationDetails" xlink:type="extended" id="ifd0281c1a366412bb985df99433a54a8_FairValueMeasurementsChangesInFairValueOfLevel3LiabilitiesRepresentingAcquisitionRelatedContingentConsiderationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_b703da6e-6042-417b-98be-eb87b52f3a08" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_6c7f9b8f-8c76-4866-bfa7-9f541a185419" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_b703da6e-6042-417b-98be-eb87b52f3a08" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_6c7f9b8f-8c76-4866-bfa7-9f541a185419" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_5d0c4180-59ca-4a43-92ec-c8cde74c09f1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_6c7f9b8f-8c76-4866-bfa7-9f541a185419" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_5d0c4180-59ca-4a43-92ec-c8cde74c09f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_ae124fa1-4dae-4e7e-961e-896a22b55994" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_6c7f9b8f-8c76-4866-bfa7-9f541a185419" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_ae124fa1-4dae-4e7e-961e-896a22b55994" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_c50cda9a-e3a4-4df4-b57c-0d1ce2c423cd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_6c7f9b8f-8c76-4866-bfa7-9f541a185419" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_c50cda9a-e3a4-4df4-b57c-0d1ce2c423cd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_8f3c1ef9-dec0-47b4-a53d-1ace409e4f5d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_6c7f9b8f-8c76-4866-bfa7-9f541a185419" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_8f3c1ef9-dec0-47b4-a53d-1ace409e4f5d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_9d16aca1-d7b8-412a-97cb-452488ae2adc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_98ffe082-5ca0-4bf4-88ce-165c3865b69d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_b703da6e-6042-417b-98be-eb87b52f3a08" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_98ffe082-5ca0-4bf4-88ce-165c3865b69d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_5efed87f-b788-42f7-92ff-264c532b1d6f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_98ffe082-5ca0-4bf4-88ce-165c3865b69d" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_5efed87f-b788-42f7-92ff-264c532b1d6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5efed87f-b788-42f7-92ff-264c532b1d6f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_5efed87f-b788-42f7-92ff-264c532b1d6f" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5efed87f-b788-42f7-92ff-264c532b1d6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_cc463597-1f99-45d6-98ba-6587e9f3efbd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_5efed87f-b788-42f7-92ff-264c532b1d6f" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_cc463597-1f99-45d6-98ba-6587e9f3efbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ContingentConsiderationMember_6dec1171-88b8-4365-9096-83223690b00e" xlink:href="mmc-20230630.xsd#mmc_ContingentConsiderationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_cc463597-1f99-45d6-98ba-6587e9f3efbd" xlink:to="loc_mmc_ContingentConsiderationMember_6dec1171-88b8-4365-9096-83223690b00e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/DerivativesDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#DerivativesDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/DerivativesDetails" xlink:type="extended" id="i6b7f7007e99a4572b25ab43821d4f85f_DerivativesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_e8826a51-39ec-4d2f-95f2-77c2d9835170" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_fbcef24c-e36e-483c-8262-258522454257" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_e8826a51-39ec-4d2f-95f2-77c2d9835170" xlink:to="loc_us-gaap_DerivativeNotionalAmount_fbcef24c-e36e-483c-8262-258522454257" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_NetInvestmentHedgeThresholdPercentageoftheEquityBalance_1ace3a8e-b434-453b-a980-fe892bcd0b07" xlink:href="mmc-20230630.xsd#mmc_NetInvestmentHedgeThresholdPercentageoftheEquityBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_e8826a51-39ec-4d2f-95f2-77c2d9835170" xlink:to="loc_mmc_NetInvestmentHedgeThresholdPercentageoftheEquityBalance_1ace3a8e-b434-453b-a980-fe892bcd0b07" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossBeforeReclassificationAndTax_e3171655-0153-452a-88a5-6bb3dd87d570" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossBeforeReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_e8826a51-39ec-4d2f-95f2-77c2d9835170" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossBeforeReclassificationAndTax_e3171655-0153-452a-88a5-6bb3dd87d570" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_54cc0e97-38a6-45a1-85a4-ae7cdb4cca5f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_e8826a51-39ec-4d2f-95f2-77c2d9835170" xlink:to="loc_us-gaap_DerivativeTable_54cc0e97-38a6-45a1-85a4-ae7cdb4cca5f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_02400385-2cf9-498c-824f-a2cfc2e81a45" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_54cc0e97-38a6-45a1-85a4-ae7cdb4cca5f" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_02400385-2cf9-498c-824f-a2cfc2e81a45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_02400385-2cf9-498c-824f-a2cfc2e81a45_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_02400385-2cf9-498c-824f-a2cfc2e81a45" xlink:to="loc_us-gaap_HedgingRelationshipDomain_02400385-2cf9-498c-824f-a2cfc2e81a45_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_f372068a-ac1d-4176-ba43-71a0a6a66a2c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_02400385-2cf9-498c-824f-a2cfc2e81a45" xlink:to="loc_us-gaap_HedgingRelationshipDomain_f372068a-ac1d-4176-ba43-71a0a6a66a2c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetInvestmentHedgingMember_b5403b97-6afc-46d8-8135-f7efd19edf74" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_NetInvestmentHedgingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_f372068a-ac1d-4176-ba43-71a0a6a66a2c" xlink:to="loc_us-gaap_NetInvestmentHedgingMember_b5403b97-6afc-46d8-8135-f7efd19edf74" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/LeasesNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#LeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/LeasesNarrativeDetails" xlink:type="extended" id="ic9ba72f0ae344d6d92e1bbd589ca08d2_LeasesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_36c58fb1-7c3b-4b7b-95cf-89076c79ecd3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_7b7fdccc-d11d-4743-821d-ad882b326818" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_36c58fb1-7c3b-4b7b-95cf-89076c79ecd3" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_7b7fdccc-d11d-4743-821d-ad882b326818" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseImpairmentLoss_290713f5-29fd-4009-850a-92765a879b4c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLeaseImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_36c58fb1-7c3b-4b7b-95cf-89076c79ecd3" xlink:to="loc_us-gaap_OperatingLeaseImpairmentLoss_290713f5-29fd-4009-850a-92765a879b4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_LesseeOperatingLeaseLeaseNotYetCommencedAmount_69c3316a-3657-4975-b284-5d406a285e84" xlink:href="mmc-20230630.xsd#mmc_LesseeOperatingLeaseLeaseNotYetCommencedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_36c58fb1-7c3b-4b7b-95cf-89076c79ecd3" xlink:to="loc_mmc_LesseeOperatingLeaseLeaseNotYetCommencedAmount_69c3316a-3657-4975-b284-5d406a285e84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_c4700ce6-78fe-4b36-ad25-a92bbac60df5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_36c58fb1-7c3b-4b7b-95cf-89076c79ecd3" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_c4700ce6-78fe-4b36-ad25-a92bbac60df5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_10ee568f-097a-4433-a90d-cc98feeb8988" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_c4700ce6-78fe-4b36-ad25-a92bbac60df5" xlink:to="loc_srt_RangeAxis_10ee568f-097a-4433-a90d-cc98feeb8988" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_10ee568f-097a-4433-a90d-cc98feeb8988_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_10ee568f-097a-4433-a90d-cc98feeb8988" xlink:to="loc_srt_RangeMember_10ee568f-097a-4433-a90d-cc98feeb8988_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a488bc73-873a-44c5-b7ce-a2e51eeab32e" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_10ee568f-097a-4433-a90d-cc98feeb8988" xlink:to="loc_srt_RangeMember_a488bc73-873a-44c5-b7ce-a2e51eeab32e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_86e20985-b5d0-41e2-8599-9ddd891a4758" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a488bc73-873a-44c5-b7ce-a2e51eeab32e" xlink:to="loc_srt_MinimumMember_86e20985-b5d0-41e2-8599-9ddd891a4758" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_9bdaa0ca-b298-4508-a33e-72b88271f38b" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a488bc73-873a-44c5-b7ce-a2e51eeab32e" xlink:to="loc_srt_MaximumMember_9bdaa0ca-b298-4508-a33e-72b88271f38b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/LeasesLeaseCostandAdditionalInformationDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#LeasesLeaseCostandAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/LeasesLeaseCostandAdditionalInformationDetails" xlink:type="extended" id="i9eb0361a5c194a3d831109cb2ba57437_LeasesLeaseCostandAdditionalInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseCost_d8b7e600-7976-4d23-bc24-506194084aae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_OperatingLeaseCost_d8b7e600-7976-4d23-bc24-506194084aae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermLeaseCost_7d260f64-3e5e-442b-95fe-5b90661ba833" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_ShortTermLeaseCost_7d260f64-3e5e-442b-95fe-5b90661ba833" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableLeaseCost_26f20ebe-081f-4c37-9c58-c3629766545e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_VariableLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_VariableLeaseCost_26f20ebe-081f-4c37-9c58-c3629766545e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubleaseIncome_4379ecb2-e443-4adc-a9f5-15c41d79d2fd" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubleaseIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_SubleaseIncome_4379ecb2-e443-4adc-a9f5-15c41d79d2fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_aeccdfa4-383c-4358-9e7d-f8720ae2fb79" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_LeaseCost_aeccdfa4-383c-4358-9e7d-f8720ae2fb79" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasePayments_0a23bca3-c749-4465-b0d8-86c67163f27c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_OperatingLeasePayments_0a23bca3-c749-4465-b0d8-86c67163f27c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_6447cbe7-51e9-4f63-8785-6fb4b0c90194" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_6447cbe7-51e9-4f63-8785-6fb4b0c90194" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_2d2bb318-371e-40c3-9fcd-264f459f7bed" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_2d2bb318-371e-40c3-9fcd-264f459f7bed" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_3ea2e3f2-7e6a-4fc7-b1c4-e23fc1c39aff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_3ea2e3f2-7e6a-4fc7-b1c4-e23fc1c39aff" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_409f6042-9051-4ede-aa58-4fbcc0ea57f4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_33eb30b1-d8e1-412b-bd37-1d1ecf4279e8" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_409f6042-9051-4ede-aa58-4fbcc0ea57f4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_cdb798ef-3127-4622-becb-be457d9f15e1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_409f6042-9051-4ede-aa58-4fbcc0ea57f4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_cdb798ef-3127-4622-becb-be457d9f15e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cdb798ef-3127-4622-becb-be457d9f15e1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_cdb798ef-3127-4622-becb-be457d9f15e1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_cdb798ef-3127-4622-becb-be457d9f15e1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3e04f02a-6f92-49e5-a8f7-9611a6ed4d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_cdb798ef-3127-4622-becb-be457d9f15e1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3e04f02a-6f92-49e5-a8f7-9611a6ed4d4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RealEstateLeaseMember_1bd629a2-c4fa-476f-b19d-8e102ed58767" xlink:href="mmc-20230630.xsd#mmc_RealEstateLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_3e04f02a-6f92-49e5-a8f7-9611a6ed4d4e" xlink:to="loc_mmc_RealEstateLeaseMember_1bd629a2-c4fa-476f-b19d-8e102ed58767" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RetirementBenefitsScheduleOfDefinedBenefitPlanWeightedAverageAssumptionUsedInCalculatingNetPeriodicBenefitCostDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RetirementBenefitsScheduleOfDefinedBenefitPlanWeightedAverageAssumptionUsedInCalculatingNetPeriodicBenefitCostDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RetirementBenefitsScheduleOfDefinedBenefitPlanWeightedAverageAssumptionUsedInCalculatingNetPeriodicBenefitCostDetails" xlink:type="extended" id="ica8e9d6507df4c86901c719a0d51b4a3_RetirementBenefitsScheduleOfDefinedBenefitPlanWeightedAverageAssumptionUsedInCalculatingNetPeriodicBenefitCostDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_03240557-8cbc-4dd2-84ec-5406fa7e6cca" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_c4ad4d35-85c7-49a3-bcfd-4348e0311359" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_03240557-8cbc-4dd2-84ec-5406fa7e6cca" xlink:to="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_c4ad4d35-85c7-49a3-bcfd-4348e0311359" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_eadd7f1b-f34f-46c2-af4a-44158cba6ba1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_c4ad4d35-85c7-49a3-bcfd-4348e0311359" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_eadd7f1b-f34f-46c2-af4a-44158cba6ba1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_3c2f1280-4e34-42a7-b175-a245250f662e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_c4ad4d35-85c7-49a3-bcfd-4348e0311359" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_3c2f1280-4e34-42a7-b175-a245250f662e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_fea90aa6-ef0f-4be7-a309-250358ddba5b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract_c4ad4d35-85c7-49a3-bcfd-4348e0311359" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_fea90aa6-ef0f-4be7-a309-250358ddba5b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_516ee58d-87d5-4d97-b1b5-ec76519449ff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_03240557-8cbc-4dd2-84ec-5406fa7e6cca" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_516ee58d-87d5-4d97-b1b5-ec76519449ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_6be54210-7622-48c1-b5f3-0417481e86a6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_516ee58d-87d5-4d97-b1b5-ec76519449ff" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_6be54210-7622-48c1-b5f3-0417481e86a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_6be54210-7622-48c1-b5f3-0417481e86a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_6be54210-7622-48c1-b5f3-0417481e86a6" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_6be54210-7622-48c1-b5f3-0417481e86a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_9b23b63e-b2cd-49b3-8ec6-78f972d6ad40" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_6be54210-7622-48c1-b5f3-0417481e86a6" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_9b23b63e-b2cd-49b3-8ec6-78f972d6ad40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_38415242-7c72-4a5a-a3f9-cccf9c7bc4cc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_9b23b63e-b2cd-49b3-8ec6-78f972d6ad40" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_38415242-7c72-4a5a-a3f9-cccf9c7bc4cc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RetirementBenefitsNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RetirementBenefitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RetirementBenefitsNarrativeDetails" xlink:type="extended" id="ifde08c3a0ba440828d1168358f480b76_RetirementBenefitsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_3f647ef5-0a1b-4983-89e3-1768762dea0c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:to="loc_us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage_3f647ef5-0a1b-4983-89e3-1768762dea0c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations_f4aa5162-a72e-4e2d-9795-1b91807e0fe0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:to="loc_us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations_f4aa5162-a72e-4e2d-9795-1b91807e0fe0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_23d06271-ad67-40b8-b6f1-0fea42622f96" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_23d06271-ad67-40b8-b6f1-0fea42622f96" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_d5f5f19a-dd77-4bef-be07-3fc2e34e3643" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanContributionsByEmployer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:to="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_d5f5f19a-dd77-4bef-be07-3fc2e34e3643" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsRemainderOfFiscalYear_5e1d31bc-1b57-49ed-9f6c-b33e8d58447b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsRemainderOfFiscalYear_5e1d31bc-1b57-49ed-9f6c-b33e8d58447b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedContributionPlanCostRecognized_f04d98d0-ef19-4495-b83f-9d69f3599356" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedContributionPlanCostRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:to="loc_us-gaap_DefinedContributionPlanCostRecognized_f04d98d0-ef19-4495-b83f-9d69f3599356" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d19ca26d-4357-4d0c-8147-d6532b493b93" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_cf2bc3d0-f891-47c9-a86f-f7c13567c313" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d19ca26d-4357-4d0c-8147-d6532b493b93" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_4a4adc97-9486-4a5a-a888-1af979a68074" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d19ca26d-4357-4d0c-8147-d6532b493b93" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_4a4adc97-9486-4a5a-a888-1af979a68074" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_4a4adc97-9486-4a5a-a888-1af979a68074_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_4a4adc97-9486-4a5a-a888-1af979a68074" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_4a4adc97-9486-4a5a-a888-1af979a68074_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_ebc3d5f7-cbc0-44b4-b811-b5478831e9b2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PlanAssetCategoriesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_4a4adc97-9486-4a5a-a888-1af979a68074" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_ebc3d5f7-cbc0-44b4-b811-b5478831e9b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityFundsMember_084ab386-b09b-4909-9933-2f79181787e8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_ebc3d5f7-cbc0-44b4-b811-b5478831e9b2" xlink:to="loc_us-gaap_EquityFundsMember_084ab386-b09b-4909-9933-2f79181787e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FixedIncomeFundsMember_e490dd1d-34b4-4256-8307-01bfb422e8f6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FixedIncomeFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanAssetCategoriesDomain_ebc3d5f7-cbc0-44b4-b811-b5478831e9b2" xlink:to="loc_us-gaap_FixedIncomeFundsMember_e490dd1d-34b4-4256-8307-01bfb422e8f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_82130b61-18b9-406c-925f-82e697d33563" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d19ca26d-4357-4d0c-8147-d6532b493b93" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_82130b61-18b9-406c-925f-82e697d33563" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_82130b61-18b9-406c-925f-82e697d33563_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_82130b61-18b9-406c-925f-82e697d33563" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_82130b61-18b9-406c-925f-82e697d33563_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_a02dc14f-05fe-4894-9cea-cd84ad178eb0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_82130b61-18b9-406c-925f-82e697d33563" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_a02dc14f-05fe-4894-9cea-cd84ad178eb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_deeaf893-4b19-4eb1-8455-bda22f79ca28" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_a02dc14f-05fe-4894-9cea-cd84ad178eb0" xlink:to="loc_us-gaap_DomesticPlanMember_deeaf893-4b19-4eb1-8455-bda22f79ca28" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_6803b181-c6a2-47cb-a4ea-7d2f95408388" xlink:href="https://xbrl.sec.gov/country/2023/country-2023.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_deeaf893-4b19-4eb1-8455-bda22f79ca28" xlink:to="loc_country_US_6803b181-c6a2-47cb-a4ea-7d2f95408388" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_a3938d7e-82f7-416e-a8dc-a17e0a7604b2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_a02dc14f-05fe-4894-9cea-cd84ad178eb0" xlink:to="loc_us-gaap_ForeignPlanMember_a3938d7e-82f7-416e-a8dc-a17e0a7604b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_61958926-7f11-48de-ab0f-f0d1d0f09695" xlink:href="https://xbrl.sec.gov/country/2023/country-2023.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignPlanMember_a3938d7e-82f7-416e-a8dc-a17e0a7604b2" xlink:to="loc_country_GB_61958926-7f11-48de-ab0f-f0d1d0f09695" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_cd0c6d3e-e79c-4bb3-91d4-95290e392374" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d19ca26d-4357-4d0c-8147-d6532b493b93" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_cd0c6d3e-e79c-4bb3-91d4-95290e392374" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_cd0c6d3e-e79c-4bb3-91d4-95290e392374_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_cd0c6d3e-e79c-4bb3-91d4-95290e392374" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_cd0c6d3e-e79c-4bb3-91d4-95290e392374_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_e837973d-f8d6-455b-91f3-9358bee2eb53" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_cd0c6d3e-e79c-4bb3-91d4-95290e392374" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_e837973d-f8d6-455b-91f3-9358bee2eb53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_f7dc3538-91d5-4f6a-8440-3d019dcf8c59" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_e837973d-f8d6-455b-91f3-9358bee2eb53" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_f7dc3538-91d5-4f6a-8440-3d019dcf8c59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ae11d0c2-da09-4ff5-a6a6-63d8b3c682aa" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d19ca26d-4357-4d0c-8147-d6532b493b93" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ae11d0c2-da09-4ff5-a6a6-63d8b3c682aa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ae11d0c2-da09-4ff5-a6a6-63d8b3c682aa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ae11d0c2-da09-4ff5-a6a6-63d8b3c682aa" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ae11d0c2-da09-4ff5-a6a6-63d8b3c682aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_28ea0acc-8f06-455a-ba7b-34617fb5868b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ae11d0c2-da09-4ff5-a6a6-63d8b3c682aa" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_28ea0acc-8f06-455a-ba7b-34617fb5868b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_UnitedKingdomPensionandPostretirementBenefitPlanDefinedBenefitMember_649ad270-285e-47d8-9ae4-0561f8c848f1" xlink:href="mmc-20230630.xsd#mmc_UnitedKingdomPensionandPostretirementBenefitPlanDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_28ea0acc-8f06-455a-ba7b-34617fb5868b" xlink:to="loc_mmc_UnitedKingdomPensionandPostretirementBenefitPlanDefinedBenefitMember_649ad270-285e-47d8-9ae4-0561f8c848f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_e03a2314-5de5-4426-bdd9-b602ffc93fa0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_d19ca26d-4357-4d0c-8147-d6532b493b93" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_e03a2314-5de5-4426-bdd9-b602ffc93fa0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_e03a2314-5de5-4426-bdd9-b602ffc93fa0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_e03a2314-5de5-4426-bdd9-b602ffc93fa0" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_e03a2314-5de5-4426-bdd9-b602ffc93fa0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_caa35f98-53d1-4e34-b02f-43168139f8ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_e03a2314-5de5-4426-bdd9-b602ffc93fa0" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_caa35f98-53d1-4e34-b02f-43168139f8ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeographicConcentrationRiskMember_f82ac751-4092-48e4-aaf9-74ebe9835c79" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GeographicConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_caa35f98-53d1-4e34-b02f-43168139f8ee" xlink:to="loc_us-gaap_GeographicConcentrationRiskMember_f82ac751-4092-48e4-aaf9-74ebe9835c79" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostCombinedUSandSignificantNonUSPlansDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostCombinedUSandSignificantNonUSPlansDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostCombinedUSandSignificantNonUSPlansDetails" xlink:type="extended" id="i9371045216d04f3bb4444573ef5dd217_RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostCombinedUSandSignificantNonUSPlansDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_40a8907d-a437-4583-b1ce-5217f070e294" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanServiceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_40a8907d-a437-4583-b1ce-5217f070e294" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_7ad9901f-294b-4491-b986-3a9ace4a9612" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_7ad9901f-294b-4491-b986-3a9ace4a9612" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_509d7b41-b07a-4460-acd1-da46ffb57995" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_509d7b41-b07a-4460-acd1-da46ffb57995" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_04526e49-73e1-40e4-adbd-5d5f4b53bb42" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_04526e49-73e1-40e4-adbd-5d5f4b53bb42" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_DefinedBenefitPlanNetPeriodicBenefitCostExcludingCurtailmentsSettlementsandPlanTerminations_e44d1ca3-20a4-49d9-92f5-644e1f8631cc" xlink:href="mmc-20230630.xsd#mmc_DefinedBenefitPlanNetPeriodicBenefitCostExcludingCurtailmentsSettlementsandPlanTerminations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_mmc_DefinedBenefitPlanNetPeriodicBenefitCostExcludingCurtailmentsSettlementsandPlanTerminations_e44d1ca3-20a4-49d9-92f5-644e1f8631cc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_3aa88556-4fe5-41a8-8eda-3248e39b5a27" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1_3aa88556-4fe5-41a8-8eda-3248e39b5a27" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_5a4d17cc-d906-479f-924e-1e4363a413bf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_5a4d17cc-d906-479f-924e-1e4363a413bf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_89646eaf-0fab-428c-8fb4-69a7512f827c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_583ad571-5298-420c-9b62-bd5a4f18d70f" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_89646eaf-0fab-428c-8fb4-69a7512f827c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_ffa5ab04-82d3-46e5-a24b-2bad374edbc8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_89646eaf-0fab-428c-8fb4-69a7512f827c" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_ffa5ab04-82d3-46e5-a24b-2bad374edbc8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_ffa5ab04-82d3-46e5-a24b-2bad374edbc8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_ffa5ab04-82d3-46e5-a24b-2bad374edbc8" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_ffa5ab04-82d3-46e5-a24b-2bad374edbc8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_98bdae58-af23-4e67-9393-50a6bd0cee4f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_ffa5ab04-82d3-46e5-a24b-2bad374edbc8" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_98bdae58-af23-4e67-9393-50a6bd0cee4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_404a598d-a0e2-404a-9cdb-52998f0b0091" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_98bdae58-af23-4e67-9393-50a6bd0cee4f" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_404a598d-a0e2-404a-9cdb-52998f0b0091" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationAxis_ba9cc821-f038-4229-bf8c-df146f80a26a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_89646eaf-0fab-428c-8fb4-69a7512f827c" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationAxis_ba9cc821-f038-4229-bf8c-df146f80a26a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_ba9cc821-f038-4229-bf8c-df146f80a26a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_ba9cc821-f038-4229-bf8c-df146f80a26a" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_ba9cc821-f038-4229-bf8c-df146f80a26a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanSponsorLocationDomain_fc15a854-808d-4602-907d-d122b35cca74" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanSponsorLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationAxis_ba9cc821-f038-4229-bf8c-df146f80a26a" xlink:to="loc_us-gaap_RetirementPlanSponsorLocationDomain_fc15a854-808d-4602-907d-d122b35cca74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticPlanMember_fcd65da7-d3ff-455a-9473-782fa397f594" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DomesticPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_fc15a854-808d-4602-907d-d122b35cca74" xlink:to="loc_us-gaap_DomesticPlanMember_fcd65da7-d3ff-455a-9473-782fa397f594" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_59af8e35-725a-4600-b17f-d71d89562146" xlink:href="https://xbrl.sec.gov/country/2023/country-2023.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticPlanMember_fcd65da7-d3ff-455a-9473-782fa397f594" xlink:to="loc_country_US_59af8e35-725a-4600-b17f-d71d89562146" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignPlanMember_3f2bd50c-311b-4634-a365-a00124f412b1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ForeignPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanSponsorLocationDomain_fc15a854-808d-4602-907d-d122b35cca74" xlink:to="loc_us-gaap_ForeignPlanMember_3f2bd50c-311b-4634-a365-a00124f412b1" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostAmountsRecordedinConsolidatedStatementofIncomeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostAmountsRecordedinConsolidatedStatementofIncomeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostAmountsRecordedinConsolidatedStatementofIncomeDetails" xlink:type="extended" id="i8807685d56164c029779b2812b54af41_RetirementBenefitsScheduleOfDefinedBenefitPlanNetPeriodicBenefitCostAmountsRecordedinConsolidatedStatementofIncomeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_e9473706-59f3-4ed4-8b14-b2fa3874cd61" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_e14824fa-5165-4c72-8873-c40d34b430a8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_e9473706-59f3-4ed4-8b14-b2fa3874cd61" xlink:to="loc_us-gaap_OperatingIncomeLoss_e14824fa-5165-4c72-8873-c40d34b430a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanOtherCosts_de8f070a-aa65-4763-aff3-88d56c60c23b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanOtherCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_e9473706-59f3-4ed4-8b14-b2fa3874cd61" xlink:to="loc_us-gaap_DefinedBenefitPlanOtherCosts_de8f070a-aa65-4763-aff3-88d56c60c23b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_2110d6cb-52d2-4a01-9378-502e9bf33fe8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_e9473706-59f3-4ed4-8b14-b2fa3874cd61" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_2110d6cb-52d2-4a01-9378-502e9bf33fe8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_2915fe80-750f-4fe8-ac37-e98e4c177271" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_e9473706-59f3-4ed4-8b14-b2fa3874cd61" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_2915fe80-750f-4fe8-ac37-e98e4c177271" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_7b85cef9-151c-47c5-a7ef-478db47bbe0c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_2915fe80-750f-4fe8-ac37-e98e4c177271" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_7b85cef9-151c-47c5-a7ef-478db47bbe0c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_7b85cef9-151c-47c5-a7ef-478db47bbe0c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_7b85cef9-151c-47c5-a7ef-478db47bbe0c" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_7b85cef9-151c-47c5-a7ef-478db47bbe0c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_580f1cec-f7f9-42b7-96fd-d98d3d53bcbf" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_7b85cef9-151c-47c5-a7ef-478db47bbe0c" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_580f1cec-f7f9-42b7-96fd-d98d3d53bcbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_ddfb2aeb-02f7-4e28-baa9-edc64cad32ee" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PensionPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_580f1cec-f7f9-42b7-96fd-d98d3d53bcbf" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_ddfb2aeb-02f7-4e28-baa9-edc64cad32ee" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/DebtScheduleOfOutstandingDebtDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#DebtScheduleOfOutstandingDebtDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/DebtScheduleOfOutstandingDebtDetails" xlink:type="extended" id="if39a1309a4294024bbea9b5fbd42c159_DebtScheduleOfOutstandingDebtDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_ad2135b6-2914-4a14-b044-e93a4f09d77b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:to="loc_us-gaap_ShortTermBorrowings_ad2135b6-2914-4a14-b044-e93a4f09d77b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_24cdce5c-63c8-431d-85e9-19e91bb1d029" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:to="loc_us-gaap_LongTermDebtCurrent_24cdce5c-63c8-431d-85e9-19e91bb1d029" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtCurrent_0593f383-da60-4d99-a777-d1917d756b1b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:to="loc_us-gaap_DebtCurrent_0593f383-da60-4d99-a777-d1917d756b1b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_2b4bafe7-0324-4026-903c-ee41b0da4a1e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:to="loc_us-gaap_LongTermDebt_2b4bafe7-0324-4026-903c-ee41b0da4a1e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_40c786e4-89e4-45cf-9fe6-ac2729b71bc7" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_40c786e4-89e4-45cf-9fe6-ac2729b71bc7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_759ef6c4-7b69-4bd7-af49-163f228e91be" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_759ef6c4-7b69-4bd7-af49-163f228e91be" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_b49a09bb-b1fb-47d7-aa48-37f4ed46389b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b795fc7a-65f2-45af-b922-237a7104127a" xlink:to="loc_us-gaap_DebtInstrumentTable_b49a09bb-b1fb-47d7-aa48-37f4ed46389b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_92a49d87-1554-41fa-9de6-297b821c4e32" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b49a09bb-b1fb-47d7-aa48-37f4ed46389b" xlink:to="loc_us-gaap_DebtInstrumentAxis_92a49d87-1554-41fa-9de6-297b821c4e32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_92a49d87-1554-41fa-9de6-297b821c4e32_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_92a49d87-1554-41fa-9de6-297b821c4e32" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_92a49d87-1554-41fa-9de6-297b821c4e32_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_92a49d87-1554-41fa-9de6-297b821c4e32" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A4.05SeniorDebtObligationsDue2023Member_fd699914-0089-4d8e-be06-a158145ae28e" xlink:href="mmc-20230630.xsd#mmc_A4.05SeniorDebtObligationsDue2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A4.05SeniorDebtObligationsDue2023Member_fd699914-0089-4d8e-be06-a158145ae28e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SeniorDebtObligationsDue2024Member_5aa20c47-3aa6-4ea0-be60-d66bc0bc5e24" xlink:href="mmc-20230630.xsd#mmc_SeniorDebtObligationsDue2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_SeniorDebtObligationsDue2024Member_5aa20c47-3aa6-4ea0-be60-d66bc0bc5e24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A3.875SeniorDebtObligationsDue2024Member_f7f45222-69d8-4f34-b699-784abf2e8d71" xlink:href="mmc-20230630.xsd#mmc_A3.875SeniorDebtObligationsDue2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A3.875SeniorDebtObligationsDue2024Member_f7f45222-69d8-4f34-b699-784abf2e8d71" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A3.50SeniorDebtObligationsDue2025Member_750cfa04-f77f-478e-a874-3ed8c79d336d" xlink:href="mmc-20230630.xsd#mmc_A3.50SeniorDebtObligationsDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A3.50SeniorDebtObligationsDue2025Member_750cfa04-f77f-478e-a874-3ed8c79d336d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A1.349SeniorDebtObligationsDue2026Member_59b43f04-e169-44cd-b51b-2adc31882470" xlink:href="mmc-20230630.xsd#mmc_A1.349SeniorDebtObligationsDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A1.349SeniorDebtObligationsDue2026Member_59b43f04-e169-44cd-b51b-2adc31882470" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A3.75SeniorDebtObligationsDue2026Member_86e6c688-dd51-4e56-a32f-47ee442ed184" xlink:href="mmc-20230630.xsd#mmc_A3.75SeniorDebtObligationsDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A3.75SeniorDebtObligationsDue2026Member_86e6c688-dd51-4e56-a32f-47ee442ed184" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SeniorDebtObligationsDue2029Member_4f345e37-1669-4329-80d2-49ad97b7ba87" xlink:href="mmc-20230630.xsd#mmc_SeniorDebtObligationsDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_SeniorDebtObligationsDue2029Member_4f345e37-1669-4329-80d2-49ad97b7ba87" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A1.979SeniorDebtObligationsDue2030Member_6fba8300-2a54-4454-8b4f-65a783e794c1" xlink:href="mmc-20230630.xsd#mmc_A1.979SeniorDebtObligationsDue2030Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A1.979SeniorDebtObligationsDue2030Member_6fba8300-2a54-4454-8b4f-65a783e794c1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A225SeniorDebtObligationsDue2030Member_1e12e41d-c40d-4e68-8977-71f81315d402" xlink:href="mmc-20230630.xsd#mmc_A225SeniorDebtObligationsDue2030Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A225SeniorDebtObligationsDue2030Member_1e12e41d-c40d-4e68-8977-71f81315d402" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A2375SeniorDebtObligationsDue2031Member_7ecd99fc-755f-42b2-a378-d961a0c96ea9" xlink:href="mmc-20230630.xsd#mmc_A2375SeniorDebtObligationsDue2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A2375SeniorDebtObligationsDue2031Member_7ecd99fc-755f-42b2-a378-d961a0c96ea9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A5750SeniorDebtObligationsDue2032Member_c026c467-a56a-48a0-b1c6-f83724904979" xlink:href="mmc-20230630.xsd#mmc_A5750SeniorDebtObligationsDue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A5750SeniorDebtObligationsDue2032Member_c026c467-a56a-48a0-b1c6-f83724904979" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SeniorDebtObligationsDue2033Member_f776149d-37cd-4a5c-a8b1-bfcbb2766831" xlink:href="mmc-20230630.xsd#mmc_SeniorDebtObligationsDue2033Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_SeniorDebtObligationsDue2033Member_f776149d-37cd-4a5c-a8b1-bfcbb2766831" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A4.75SeniorDebtObligationsDue2039Member_e9b9ea3f-14ab-4f0d-9050-790da6122d0e" xlink:href="mmc-20230630.xsd#mmc_A4.75SeniorDebtObligationsDue2039Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A4.75SeniorDebtObligationsDue2039Member_e9b9ea3f-14ab-4f0d-9050-790da6122d0e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SeniorDebtObligationDue2047Member_4457f3d5-d5f0-45df-9965-30dc75c9592a" xlink:href="mmc-20230630.xsd#mmc_SeniorDebtObligationDue2047Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_SeniorDebtObligationDue2047Member_4457f3d5-d5f0-45df-9965-30dc75c9592a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SeniorDebtObligationsDue2048Member_46109521-027e-4cfb-a3ff-940656b28145" xlink:href="mmc-20230630.xsd#mmc_SeniorDebtObligationsDue2048Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_SeniorDebtObligationsDue2048Member_46109521-027e-4cfb-a3ff-940656b28145" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A4.90SeniorDebtObligationsDue2049Member_2417b391-a2fd-4d7c-9158-667043924d88" xlink:href="mmc-20230630.xsd#mmc_A4.90SeniorDebtObligationsDue2049Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A4.90SeniorDebtObligationsDue2049Member_2417b391-a2fd-4d7c-9158-667043924d88" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A290SeniorDebtObligationsDue2051Member_8bff16b7-81af-4140-abcb-bb0a679b5566" xlink:href="mmc-20230630.xsd#mmc_A290SeniorDebtObligationsDue2051Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A290SeniorDebtObligationsDue2051Member_8bff16b7-81af-4140-abcb-bb0a679b5566" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A625SeniorDebtObligationsDue2052Member_58b83d2f-edae-44f0-b7ad-5f6e5f693201" xlink:href="mmc-20230630.xsd#mmc_A625SeniorDebtObligationsDue2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A625SeniorDebtObligationsDue2052Member_58b83d2f-edae-44f0-b7ad-5f6e5f693201" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A545SeniorDebtObligationsDue2053Member_f417157a-f653-4488-87c6-dbe7487ad762" xlink:href="mmc-20230630.xsd#mmc_A545SeniorDebtObligationsDue2053Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_A545SeniorDebtObligationsDue2053Member_f417157a-f653-4488-87c6-dbe7487ad762" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MortgageDue2035Member_f694c4b1-bfc2-4198-a895-3f5746c801e1" xlink:href="mmc-20230630.xsd#mmc_MortgageDue2035Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_MortgageDue2035Member_f694c4b1-bfc2-4198-a895-3f5746c801e1" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_OtherDebtInstrumentsMember_bfa96d40-b427-4b54-90e2-54ad69e3d45d" xlink:href="mmc-20230630.xsd#mmc_OtherDebtInstrumentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_991d1306-9c85-476e-abc4-0b0ddd2517e3" xlink:to="loc_mmc_OtherDebtInstrumentsMember_bfa96d40-b427-4b54-90e2-54ad69e3d45d" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_d7e326e9-c055-4acf-8b6b-92026d596258" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b49a09bb-b1fb-47d7-aa48-37f4ed46389b" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_d7e326e9-c055-4acf-8b6b-92026d596258" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_d7e326e9-c055-4acf-8b6b-92026d596258_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_d7e326e9-c055-4acf-8b6b-92026d596258" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_d7e326e9-c055-4acf-8b6b-92026d596258_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_312ca404-0e1b-4b18-9580-76a816d7e3f3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_d7e326e9-c055-4acf-8b6b-92026d596258" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_312ca404-0e1b-4b18-9580-76a816d7e3f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_0b97eaf8-f7eb-4ea1-a8a3-7e8a80a4398a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_312ca404-0e1b-4b18-9580-76a816d7e3f3" xlink:to="loc_us-gaap_CommercialPaperMember_0b97eaf8-f7eb-4ea1-a8a3-7e8a80a4398a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_28e7de8c-a314-43e2-b2e7-365690edf08a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_312ca404-0e1b-4b18-9580-76a816d7e3f3" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_28e7de8c-a314-43e2-b2e7-365690edf08a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/DebtNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#DebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/DebtNarrativeDetails" xlink:type="extended" id="i61ef1a6c3150469a912583b32353e525_DebtNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_3d16b1a9-29ce-438e-959d-71a2681b5a5c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_ShortTermBorrowings_3d16b1a9-29ce-438e-959d-71a2681b5a5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaper_eac83adf-f25f-4c24-bcd2-9a85fc18e286" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommercialPaper"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_CommercialPaper_eac83adf-f25f-4c24-bcd2-9a85fc18e286" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_f4979748-09fb-448e-b24a-19e08ee0edb5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_f4979748-09fb-448e-b24a-19e08ee0edb5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_6e3972ee-f656-4c03-a360-19b796cbfe08" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_6e3972ee-f656-4c03-a360-19b796cbfe08" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_ff4b4b07-3a60-4015-9507-af4695c0f186" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_DebtInstrumentTerm_ff4b4b07-3a60-4015-9507-af4695c0f186" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_37dc1a74-67cc-4e9c-814c-f1b615fa4441" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_LineOfCredit_37dc1a74-67cc-4e9c-814c-f1b615fa4441" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtWeightedAverageInterestRate_1b191cde-d20e-4344-89a3-f11ece03cf47" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_DebtWeightedAverageInterestRate_1b191cde-d20e-4344-89a3-f11ece03cf47" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForOtherCreditLosses_21409aa6-1d13-4b38-b28f-59fda1c6646a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ProvisionForOtherCreditLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_ProvisionForOtherCreditLosses_21409aa6-1d13-4b38-b28f-59fda1c6646a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_73cc41d6-405c-4937-a166-70f380eb5fb8" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_73cc41d6-405c-4937-a166-70f380eb5fb8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_2ebd0aa3-b48e-49ca-8cba-7d7e3d54b835" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_2ebd0aa3-b48e-49ca-8cba-7d7e3d54b835" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_76a6abd2-9ebc-474a-901c-fa4c04b364e1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_76a6abd2-9ebc-474a-901c-fa4c04b364e1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfDebt_cd9b26d3-84b5-4d71-b9b6-4da4e5132fca" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RepaymentsOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_RepaymentsOfDebt_cd9b26d3-84b5-4d71-b9b6-4da4e5132fca" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_29a2f4d1-ea6f-48d4-9b48-da2f1914dbd9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_04e20268-ab51-4ce6-a666-d5183e1c990e" xlink:to="loc_us-gaap_DebtInstrumentTable_29a2f4d1-ea6f-48d4-9b48-da2f1914dbd9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_d783c769-91c4-4845-afe2-81238c6f29a3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_29a2f4d1-ea6f-48d4-9b48-da2f1914dbd9" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_d783c769-91c4-4845-afe2-81238c6f29a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_d783c769-91c4-4845-afe2-81238c6f29a3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d783c769-91c4-4845-afe2-81238c6f29a3" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_d783c769-91c4-4845-afe2-81238c6f29a3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_1d8b5892-59fc-4fad-9788-9b87a374db15" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d783c769-91c4-4845-afe2-81238c6f29a3" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_1d8b5892-59fc-4fad-9788-9b87a374db15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_d7d82bee-eb67-4450-9831-3736f71f5056" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_1d8b5892-59fc-4fad-9788-9b87a374db15" xlink:to="loc_us-gaap_SeniorNotesMember_d7d82bee-eb67-4450-9831-3736f71f5056" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_1fce58c1-e228-48b1-a469-00729bb3c04c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_29a2f4d1-ea6f-48d4-9b48-da2f1914dbd9" xlink:to="loc_us-gaap_CreditFacilityAxis_1fce58c1-e228-48b1-a469-00729bb3c04c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_1fce58c1-e228-48b1-a469-00729bb3c04c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_1fce58c1-e228-48b1-a469-00729bb3c04c" xlink:to="loc_us-gaap_CreditFacilityDomain_1fce58c1-e228-48b1-a469-00729bb3c04c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_2ae25bfd-5d7c-48f5-bc11-d0a301caf54e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_1fce58c1-e228-48b1-a469-00729bb3c04c" xlink:to="loc_us-gaap_CreditFacilityDomain_2ae25bfd-5d7c-48f5-bc11-d0a301caf54e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_9b478478-57e7-48e7-8cf6-eaffc9a53bb6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_2ae25bfd-5d7c-48f5-bc11-d0a301caf54e" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_9b478478-57e7-48e7-8cf6-eaffc9a53bb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_3c1541be-2648-40ab-b904-a669e38263ec" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_29a2f4d1-ea6f-48d4-9b48-da2f1914dbd9" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_3c1541be-2648-40ab-b904-a669e38263ec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_3c1541be-2648-40ab-b904-a669e38263ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_3c1541be-2648-40ab-b904-a669e38263ec" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_3c1541be-2648-40ab-b904-a669e38263ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_7b4d03d5-4c4d-49a0-898a-a43f6778df2d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_3c1541be-2648-40ab-b904-a669e38263ec" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_7b4d03d5-4c4d-49a0-898a-a43f6778df2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_9342463c-0e6a-4a87-8ab8-3610133eb374" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_7b4d03d5-4c4d-49a0-898a-a43f6778df2d" xlink:to="loc_us-gaap_CommercialPaperMember_9342463c-0e6a-4a87-8ab8-3610133eb374" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_9814b4d4-c46e-482e-b84e-c7bb7a5b1a64" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_7b4d03d5-4c4d-49a0-898a-a43f6778df2d" xlink:to="loc_us-gaap_LineOfCreditMember_9814b4d4-c46e-482e-b84e-c7bb7a5b1a64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_c532f4b2-2a0d-4efd-9573-af4cc9a1be0f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_29a2f4d1-ea6f-48d4-9b48-da2f1914dbd9" xlink:to="loc_us-gaap_DebtInstrumentAxis_c532f4b2-2a0d-4efd-9573-af4cc9a1be0f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c532f4b2-2a0d-4efd-9573-af4cc9a1be0f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_c532f4b2-2a0d-4efd-9573-af4cc9a1be0f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c532f4b2-2a0d-4efd-9573-af4cc9a1be0f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_c532f4b2-2a0d-4efd-9573-af4cc9a1be0f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_NewFacilityMember_7685eafb-bdd3-4d64-9125-b3d1cae6e9f2" xlink:href="mmc-20230630.xsd#mmc_NewFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_NewFacilityMember_7685eafb-bdd3-4d64-9125-b3d1cae6e9f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_UncommittedCreditFacilityMember_d9a28e6e-1411-437f-ba0a-85276c5d6730" xlink:href="mmc-20230630.xsd#mmc_UncommittedCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_UncommittedCreditFacilityMember_d9a28e6e-1411-437f-ba0a-85276c5d6730" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_AmendedRevolvingCreditFacilityMember_467a70fa-0f9f-473a-9221-be9836ee8949" xlink:href="mmc-20230630.xsd#mmc_AmendedRevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_AmendedRevolvingCreditFacilityMember_467a70fa-0f9f-473a-9221-be9836ee8949" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_OtherDebtFacilitiesMember_03aea4b9-fb67-4096-a4b4-e74c7672c943" xlink:href="mmc-20230630.xsd#mmc_OtherDebtFacilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_OtherDebtFacilitiesMember_03aea4b9-fb67-4096-a4b4-e74c7672c943" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A545SeniorDebtObligationsDue2053Member_df4d772f-adf9-4ac9-9b55-0e18f0976c59" xlink:href="mmc-20230630.xsd#mmc_A545SeniorDebtObligationsDue2053Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_A545SeniorDebtObligationsDue2053Member_df4d772f-adf9-4ac9-9b55-0e18f0976c59" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A5750SeniorDebtObligationsDue2032Member_1efca9d9-dd31-4391-afa6-96f14c4bb94a" xlink:href="mmc-20230630.xsd#mmc_A5750SeniorDebtObligationsDue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_A5750SeniorDebtObligationsDue2032Member_1efca9d9-dd31-4391-afa6-96f14c4bb94a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A625SeniorDebtObligationsDue2052Member_8ff453b3-a16e-4355-b968-24d4f1bb9961" xlink:href="mmc-20230630.xsd#mmc_A625SeniorDebtObligationsDue2052Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_A625SeniorDebtObligationsDue2052Member_8ff453b3-a16e-4355-b968-24d4f1bb9961" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_A330SeniorDebtObligationsDueMarch2023Member_3a6033b5-ce7a-4127-b51f-f52bdc31b667" xlink:href="mmc-20230630.xsd#mmc_A330SeniorDebtObligationsDueMarch2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_e2532f30-b9d8-4a9c-b204-adc3d03eba73" xlink:to="loc_mmc_A330SeniorDebtObligationsDueMarch2023Member_3a6033b5-ce7a-4127-b51f-f52bdc31b667" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/DebtEstimatedFairValueofSignificantFinancialInstrumentsDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#DebtEstimatedFairValueofSignificantFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/DebtEstimatedFairValueofSignificantFinancialInstrumentsDetails" xlink:type="extended" id="i1207d5af1d4f43e1aaae654795d504fa_DebtEstimatedFairValueofSignificantFinancialInstrumentsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_105f2bf0-5f40-4903-9c0c-f962da71f1c6" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShorttermDebtFairValue_808b60fb-da53-4229-93aa-aced4c0ce470" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShorttermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_105f2bf0-5f40-4903-9c0c-f962da71f1c6" xlink:to="loc_us-gaap_ShorttermDebtFairValue_808b60fb-da53-4229-93aa-aced4c0ce470" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_36f03c3b-236f-437b-937d-e7f6bf934a24" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_105f2bf0-5f40-4903-9c0c-f962da71f1c6" xlink:to="loc_us-gaap_LongTermDebtFairValue_36f03c3b-236f-437b-937d-e7f6bf934a24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_1e6c4118-87cd-4014-a743-0dd27e0c8054" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_105f2bf0-5f40-4903-9c0c-f962da71f1c6" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_1e6c4118-87cd-4014-a743-0dd27e0c8054" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_99a9f653-85ce-452a-b014-699498c13045" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_1e6c4118-87cd-4014-a743-0dd27e0c8054" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_99a9f653-85ce-452a-b014-699498c13045" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_99a9f653-85ce-452a-b014-699498c13045_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_99a9f653-85ce-452a-b014-699498c13045" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_99a9f653-85ce-452a-b014-699498c13045_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_e82ea032-c6ab-407d-884b-922deb11792a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_99a9f653-85ce-452a-b014-699498c13045" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_e82ea032-c6ab-407d-884b-922deb11792a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a96d262f-9f9f-4918-92e1-3fd83dee5873" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_e82ea032-c6ab-407d-884b-922deb11792a" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a96d262f-9f9f-4918-92e1-3fd83dee5873" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_71786d42-4ab0-495d-aee8-a2af09626b74" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a96d262f-9f9f-4918-92e1-3fd83dee5873" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_71786d42-4ab0-495d-aee8-a2af09626b74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_30df66ab-659f-44e0-bb6f-15eacaed05b3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_a96d262f-9f9f-4918-92e1-3fd83dee5873" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_30df66ab-659f-44e0-bb6f-15eacaed05b3" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RestructuringCostsNarrativeDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RestructuringCostsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RestructuringCostsNarrativeDetails" xlink:type="extended" id="ic8c379cea2d74531b65a8b7b7d0f99ef_RestructuringCostsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_1f49fd47-5da8-4ee4-a9df-49e0e7f74aa3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedCostRemaining1_cf8d2bcf-fc49-48c7-a90d-fcc8b4a8f72c" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringAndRelatedCostExpectedCostRemaining1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1f49fd47-5da8-4ee4-a9df-49e0e7f74aa3" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedCostRemaining1_cf8d2bcf-fc49-48c7-a90d-fcc8b4a8f72c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostCostIncurredToDate1_3828a46a-dc78-4d2e-89cb-8b8faa0ea4f4" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringAndRelatedCostCostIncurredToDate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1f49fd47-5da8-4ee4-a9df-49e0e7f74aa3" xlink:to="loc_us-gaap_RestructuringAndRelatedCostCostIncurredToDate1_3828a46a-dc78-4d2e-89cb-8b8faa0ea4f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_ff66216b-1f63-4100-bdaf-c3b6e0eb140e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1f49fd47-5da8-4ee4-a9df-49e0e7f74aa3" xlink:to="loc_us-gaap_RestructuringCharges_ff66216b-1f63-4100-bdaf-c3b6e0eb140e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_df6d2fd1-94e3-4b0e-98a9-211a4203b90b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1f49fd47-5da8-4ee4-a9df-49e0e7f74aa3" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_df6d2fd1-94e3-4b0e-98a9-211a4203b90b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_0a254527-cc61-4b04-ba8f-ffe444a5e548" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_df6d2fd1-94e3-4b0e-98a9-211a4203b90b" xlink:to="loc_srt_RangeAxis_0a254527-cc61-4b04-ba8f-ffe444a5e548" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_0a254527-cc61-4b04-ba8f-ffe444a5e548_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_0a254527-cc61-4b04-ba8f-ffe444a5e548" xlink:to="loc_srt_RangeMember_0a254527-cc61-4b04-ba8f-ffe444a5e548_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_25f6121c-fd32-487d-a579-3b4c4fb401b6" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_0a254527-cc61-4b04-ba8f-ffe444a5e548" xlink:to="loc_srt_RangeMember_25f6121c-fd32-487d-a579-3b4c4fb401b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_8a101c39-4017-4b17-8710-cce066ec21f2" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_25f6121c-fd32-487d-a579-3b4c4fb401b6" xlink:to="loc_srt_MinimumMember_8a101c39-4017-4b17-8710-cce066ec21f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_d24bb610-008c-4372-b80d-0da6a2e1e073" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_25f6121c-fd32-487d-a579-3b4c4fb401b6" xlink:to="loc_srt_MaximumMember_d24bb610-008c-4372-b80d-0da6a2e1e073" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_56d0d52f-73d4-4f9c-a53d-0ecca576c35e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_df6d2fd1-94e3-4b0e-98a9-211a4203b90b" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_56d0d52f-73d4-4f9c-a53d-0ecca576c35e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_56d0d52f-73d4-4f9c-a53d-0ecca576c35e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_56d0d52f-73d4-4f9c-a53d-0ecca576c35e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_56d0d52f-73d4-4f9c-a53d-0ecca576c35e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_2f83c6b5-bace-48a1-afb2-d79d05a278af" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_56d0d52f-73d4-4f9c-a53d-0ecca576c35e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_2f83c6b5-bace-48a1-afb2-d79d05a278af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SeveranceAndLeaseExitChargesMember_780a38f7-b913-4322-87db-36262ed0d285" xlink:href="mmc-20230630.xsd#mmc_SeveranceAndLeaseExitChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_2f83c6b5-bace-48a1-afb2-d79d05a278af" xlink:to="loc_mmc_SeveranceAndLeaseExitChargesMember_780a38f7-b913-4322-87db-36262ed0d285" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RestructuringCostsRestructuringandRelatedCostsIncurredDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RestructuringCostsRestructuringandRelatedCostsIncurredDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RestructuringCostsRestructuringandRelatedCostsIncurredDetails" xlink:type="extended" id="idf2185b804e843f080706bf109fd97b5_RestructuringCostsRestructuringandRelatedCostsIncurredDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_bc6bda08-93c4-4eb8-a088-1243c3ec67e5" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_e01b1b92-cb99-45ff-bcdf-0678b88decd2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_bc6bda08-93c4-4eb8-a088-1243c3ec67e5" xlink:to="loc_us-gaap_RestructuringCharges_e01b1b92-cb99-45ff-bcdf-0678b88decd2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_07f9f926-57ee-4b30-8f31-a3b595f56c20" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_bc6bda08-93c4-4eb8-a088-1243c3ec67e5" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_07f9f926-57ee-4b30-8f31-a3b595f56c20" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_90ade1de-908c-4e30-b3f3-1d7a8c79620b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_07f9f926-57ee-4b30-8f31-a3b595f56c20" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_90ade1de-908c-4e30-b3f3-1d7a8c79620b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_90ade1de-908c-4e30-b3f3-1d7a8c79620b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_90ade1de-908c-4e30-b3f3-1d7a8c79620b" xlink:to="loc_us-gaap_SegmentDomain_90ade1de-908c-4e30-b3f3-1d7a8c79620b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_af9abdc6-bbd6-4e1d-964d-66ec8f4b0380" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_90ade1de-908c-4e30-b3f3-1d7a8c79620b" xlink:to="loc_us-gaap_SegmentDomain_af9abdc6-bbd6-4e1d-964d-66ec8f4b0380" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RiskandInsuranceServicesSegmentMember_5dfc43ad-6061-4b08-8989-0131cc80d4d4" xlink:href="mmc-20230630.xsd#mmc_RiskandInsuranceServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_af9abdc6-bbd6-4e1d-964d-66ec8f4b0380" xlink:to="loc_mmc_RiskandInsuranceServicesSegmentMember_5dfc43ad-6061-4b08-8989-0131cc80d4d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ConsultingSegmentMember_47540db5-686e-4bb8-aad9-99e7a5a40132" xlink:href="mmc-20230630.xsd#mmc_ConsultingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_af9abdc6-bbd6-4e1d-964d-66ec8f4b0380" xlink:to="loc_mmc_ConsultingSegmentMember_47540db5-686e-4bb8-aad9-99e7a5a40132" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_f888095a-a0a2-486c-b06d-9a72c15e55ba" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_07f9f926-57ee-4b30-8f31-a3b595f56c20" xlink:to="loc_srt_ConsolidationItemsAxis_f888095a-a0a2-486c-b06d-9a72c15e55ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_f888095a-a0a2-486c-b06d-9a72c15e55ba_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_f888095a-a0a2-486c-b06d-9a72c15e55ba" xlink:to="loc_srt_ConsolidationItemsDomain_f888095a-a0a2-486c-b06d-9a72c15e55ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_8e3583e6-be15-4edd-bc02-b7c49947d8ec" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_f888095a-a0a2-486c-b06d-9a72c15e55ba" xlink:to="loc_srt_ConsolidationItemsDomain_8e3583e6-be15-4edd-bc02-b7c49947d8ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_f1fc4043-ba70-4dfa-9572-3ed885f99357" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_8e3583e6-be15-4edd-bc02-b7c49947d8ec" xlink:to="loc_us-gaap_CorporateNonSegmentMember_f1fc4043-ba70-4dfa-9572-3ed885f99357" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/RestructuringCostsRestructuringActivitiesDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#RestructuringCostsRestructuringActivitiesDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/RestructuringCostsRestructuringActivitiesDetails" xlink:type="extended" id="i221c19cb5eb849bdb937700de9aeb57b_RestructuringCostsRestructuringActivitiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_1f2bba47-a1c7-4311-8067-c817db28775a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveRollForward_d2b5378c-342a-412c-b614-182007f9c536" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringReserveRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1f2bba47-a1c7-4311-8067-c817db28775a" xlink:to="loc_us-gaap_RestructuringReserveRollForward_d2b5378c-342a-412c-b614-182007f9c536" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_42132639-54c1-4332-a602-c55936aecd6f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_d2b5378c-342a-412c-b614-182007f9c536" xlink:to="loc_us-gaap_RestructuringReserve_42132639-54c1-4332-a602-c55936aecd6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_c240b227-c150-4223-a9cf-2521be9ca0b3" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_d2b5378c-342a-412c-b614-182007f9c536" xlink:to="loc_us-gaap_RestructuringCharges_c240b227-c150-4223-a9cf-2521be9ca0b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRestructuring_cfff9e9b-72f4-4c69-a922-1dd0386fceff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_PaymentsForRestructuring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_d2b5378c-342a-412c-b614-182007f9c536" xlink:to="loc_us-gaap_PaymentsForRestructuring_cfff9e9b-72f4-4c69-a922-1dd0386fceff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveSettledWithoutCash2_a8d1efbf-2c56-4cc5-bd2e-a608dac8495f" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringReserveSettledWithoutCash2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_d2b5378c-342a-412c-b614-182007f9c536" xlink:to="loc_us-gaap_RestructuringReserveSettledWithoutCash2_a8d1efbf-2c56-4cc5-bd2e-a608dac8495f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserve_8c0572a8-fa4b-4de0-805e-ad5b690fd1cc" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringReserve"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_cba3b987-cd97-4a43-b94f-31e1c29accae" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_1f2bba47-a1c7-4311-8067-c817db28775a" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_cba3b987-cd97-4a43-b94f-31e1c29accae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_1ffaf443-1c4c-4e13-b3fb-ec8b2625dd81" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_cba3b987-cd97-4a43-b94f-31e1c29accae" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_1ffaf443-1c4c-4e13-b3fb-ec8b2625dd81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1ffaf443-1c4c-4e13-b3fb-ec8b2625dd81_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1ffaf443-1c4c-4e13-b3fb-ec8b2625dd81" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_1ffaf443-1c4c-4e13-b3fb-ec8b2625dd81_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d054964d-2e5e-40cc-8e3f-1ea607c534c0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_1ffaf443-1c4c-4e13-b3fb-ec8b2625dd81" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d054964d-2e5e-40cc-8e3f-1ea607c534c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_JardineLloydThompsonGroupplcMember_ff94ab81-d27d-4c9c-b65a-d6386c1789ca" xlink:href="mmc-20230630.xsd#mmc_JardineLloydThompsonGroupplcMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d054964d-2e5e-40cc-8e3f-1ea607c534c0" xlink:to="loc_mmc_JardineLloydThompsonGroupplcMember_ff94ab81-d27d-4c9c-b65a-d6386c1789ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_21695c2f-79d1-4df3-9377-1c485178a260" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_cba3b987-cd97-4a43-b94f-31e1c29accae" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_21695c2f-79d1-4df3-9377-1c485178a260" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_21695c2f-79d1-4df3-9377-1c485178a260_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_21695c2f-79d1-4df3-9377-1c485178a260" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_21695c2f-79d1-4df3-9377-1c485178a260_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_20ecd928-a007-4ce5-a33c-0a5fc5453e9a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_21695c2f-79d1-4df3-9377-1c485178a260" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_20ecd928-a007-4ce5-a33c-0a5fc5453e9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_91e186b7-8052-404e-901f-5f3922df4aed" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_20ecd928-a007-4ce5-a33c-0a5fc5453e9a" xlink:to="loc_us-gaap_EmployeeSeveranceMember_91e186b7-8052-404e-901f-5f3922df4aed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RealEstateRelatedCostsMember_b8594303-ff17-4240-b7ac-d5b4de157a2e" xlink:href="mmc-20230630.xsd#mmc_RealEstateRelatedCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_20ecd928-a007-4ce5-a33c-0a5fc5453e9a" xlink:to="loc_mmc_RealEstateRelatedCostsMember_b8594303-ff17-4240-b7ac-d5b4de157a2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_InformationTechnologyMember_c8b319e0-ed5d-4af0-a521-23b24918b193" xlink:href="mmc-20230630.xsd#mmc_InformationTechnologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_20ecd928-a007-4ce5-a33c-0a5fc5453e9a" xlink:to="loc_mmc_InformationTechnologyMember_c8b319e0-ed5d-4af0-a521-23b24918b193" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_LegalConsultingandOtherOutsideServicesMember_9f8a6c9a-3706-4dc4-b35f-0f18fc2f8a36" xlink:href="mmc-20230630.xsd#mmc_LegalConsultingandOtherOutsideServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_20ecd928-a007-4ce5-a33c-0a5fc5453e9a" xlink:to="loc_mmc_LegalConsultingandOtherOutsideServicesMember_9f8a6c9a-3706-4dc4-b35f-0f18fc2f8a36" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/CommonStockDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#CommonStockDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/CommonStockDetails" xlink:type="extended" id="if1fa3c2f4ded49c4aafef900755ecd9b_CommonStockDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityClassOfTreasuryStockLineItems_66d9ecd5-4766-4189-a8d9-b72f96f470b9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EquityClassOfTreasuryStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_e6e292b8-b2e8-4ccc-9b1a-92a075e5ae73" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_66d9ecd5-4766-4189-a8d9-b72f96f470b9" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_e6e292b8-b2e8-4ccc-9b1a-92a075e5ae73" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_16d64a59-0ec4-44b4-8295-dae07ef03994" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_66d9ecd5-4766-4189-a8d9-b72f96f470b9" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_16d64a59-0ec4-44b4-8295-dae07ef03994" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_57bd94cd-4a5d-4a4a-b1e7-34bbb07e3e00" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_66d9ecd5-4766-4189-a8d9-b72f96f470b9" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_57bd94cd-4a5d-4a4a-b1e7-34bbb07e3e00" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_ec12bac5-2239-4331-ab96-a05b88cfcf99" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_66d9ecd5-4766-4189-a8d9-b72f96f470b9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_ec12bac5-2239-4331-ab96-a05b88cfcf99" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_8fd5cc66-580c-438f-b0ec-a32ae42c00ef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockDividendsPerShareDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_66d9ecd5-4766-4189-a8d9-b72f96f470b9" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_8fd5cc66-580c-438f-b0ec-a32ae42c00ef" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfTreasuryStockTable_faf9630d-e789-4c84-ace7-c23110e066d9" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfTreasuryStockTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EquityClassOfTreasuryStockLineItems_66d9ecd5-4766-4189-a8d9-b72f96f470b9" xlink:to="loc_us-gaap_ClassOfTreasuryStockTable_faf9630d-e789-4c84-ace7-c23110e066d9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_a6042625-9f59-45be-97a6-52a944070d7b" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_faf9630d-e789-4c84-ace7-c23110e066d9" xlink:to="loc_us-gaap_StatementClassOfStockAxis_a6042625-9f59-45be-97a6-52a944070d7b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_a6042625-9f59-45be-97a6-52a944070d7b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_a6042625-9f59-45be-97a6-52a944070d7b" xlink:to="loc_us-gaap_ClassOfStockDomain_a6042625-9f59-45be-97a6-52a944070d7b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_2ac9cb8c-6a25-4332-a204-c2fa5eb79249" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_a6042625-9f59-45be-97a6-52a944070d7b" xlink:to="loc_us-gaap_ClassOfStockDomain_2ac9cb8c-6a25-4332-a204-c2fa5eb79249" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_584d5073-9351-4a3b-b54a-3cba99b49cf2" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_2ac9cb8c-6a25-4332-a204-c2fa5eb79249" xlink:to="loc_us-gaap_CommonStockMember_584d5073-9351-4a3b-b54a-3cba99b49cf2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_4824d616-cf86-487d-805d-248d8770cbff" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfTreasuryStockTable_faf9630d-e789-4c84-ace7-c23110e066d9" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_4824d616-cf86-487d-805d-248d8770cbff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_4824d616-cf86-487d-805d-248d8770cbff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_4824d616-cf86-487d-805d-248d8770cbff" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_4824d616-cf86-487d-805d-248d8770cbff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_0f560fb2-e8f7-4340-9c39-1a68b7cc69ba" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_4824d616-cf86-487d-805d-248d8770cbff" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_0f560fb2-e8f7-4340-9c39-1a68b7cc69ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_5a1a6ad0-3560-4fd8-b96c-8e0a21f66907" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_0f560fb2-e8f7-4340-9c39-1a68b7cc69ba" xlink:to="loc_us-gaap_SubsequentEventMember_5a1a6ad0-3560-4fd8-b96c-8e0a21f66907" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/SegmentInformationSelectedInformationAndDetailsForMMCsOperatingSegmentsDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#SegmentInformationSelectedInformationAndDetailsForMMCsOperatingSegmentsDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/SegmentInformationSelectedInformationAndDetailsForMMCsOperatingSegmentsDetails" xlink:type="extended" id="idaa160c36e6748b5b1e1a79c4c9a27e8_SegmentInformationSelectedInformationAndDetailsForMMCsOperatingSegmentsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_1e401e8d-fc2f-4b13-bd43-e51c2479b866" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_1e401e8d-fc2f-4b13-bd43-e51c2479b866" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_e9e6dc4d-7d7e-44c5-a1d5-5c5962eb0e93" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:to="loc_us-gaap_OperatingIncomeLoss_e9e6dc4d-7d7e-44c5-a1d5-5c5962eb0e93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_InterestIncomeFiduciaryAssets_62cae30f-caeb-4653-97c5-35d8cae8b82f" xlink:href="mmc-20230630.xsd#mmc_InterestIncomeFiduciaryAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:to="loc_mmc_InterestIncomeFiduciaryAssets_62cae30f-caeb-4653-97c5-35d8cae8b82f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_97ce4cfb-fe7c-4ee8-a70d-f94c4035e7e0" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_97ce4cfb-fe7c-4ee8-a70d-f94c4035e7e0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfBusiness_966b28de-f7e2-4e86-8432-7117c17b4471" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_GainLossOnSaleOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:to="loc_us-gaap_GainLossOnSaleOfBusiness_966b28de-f7e2-4e86-8432-7117c17b4471" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_GainLossOnConsolidationOfBusiness_4d8fb702-afb4-4a2d-b8e9-2906265c34fc" xlink:href="mmc-20230630.xsd#mmc_GainLossOnConsolidationOfBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:to="loc_mmc_GainLossOnConsolidationOfBusiness_4d8fb702-afb4-4a2d-b8e9-2906265c34fc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_18d9150e-1d0b-49e5-b52e-2a936df65345" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6354a1f2-9c68-492e-af95-0f7f77c54176" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_18d9150e-1d0b-49e5-b52e-2a936df65345" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_b53f30f6-20e3-46b9-980f-043307ab8633" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_18d9150e-1d0b-49e5-b52e-2a936df65345" xlink:to="loc_srt_ConsolidationItemsAxis_b53f30f6-20e3-46b9-980f-043307ab8633" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_b53f30f6-20e3-46b9-980f-043307ab8633_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_b53f30f6-20e3-46b9-980f-043307ab8633" xlink:to="loc_srt_ConsolidationItemsDomain_b53f30f6-20e3-46b9-980f-043307ab8633_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_06cab290-07dc-45b1-b381-5eca93c78264" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_b53f30f6-20e3-46b9-980f-043307ab8633" xlink:to="loc_srt_ConsolidationItemsDomain_06cab290-07dc-45b1-b381-5eca93c78264" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_b893b7c3-2dea-4253-a218-7f3888a62934" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_06cab290-07dc-45b1-b381-5eca93c78264" xlink:to="loc_us-gaap_OperatingSegmentsMember_b893b7c3-2dea-4253-a218-7f3888a62934" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_CorporateAndEliminationsMember_4cccb525-dc7d-480f-a52a-a9d7df729326" xlink:href="mmc-20230630.xsd#mmc_CorporateAndEliminationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_06cab290-07dc-45b1-b381-5eca93c78264" xlink:to="loc_mmc_CorporateAndEliminationsMember_4cccb525-dc7d-480f-a52a-a9d7df729326" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_c5c3804d-80b0-4b37-a7cf-a4146f720d64" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_06cab290-07dc-45b1-b381-5eca93c78264" xlink:to="loc_us-gaap_IntersegmentEliminationMember_c5c3804d-80b0-4b37-a7cf-a4146f720d64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_34ca48b0-b093-4db7-b112-d0713ff61508" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_18d9150e-1d0b-49e5-b52e-2a936df65345" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_34ca48b0-b093-4db7-b112-d0713ff61508" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_34ca48b0-b093-4db7-b112-d0713ff61508_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_34ca48b0-b093-4db7-b112-d0713ff61508" xlink:to="loc_us-gaap_SegmentDomain_34ca48b0-b093-4db7-b112-d0713ff61508_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_44528d71-d681-4feb-aa8f-615544225e0d" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_34ca48b0-b093-4db7-b112-d0713ff61508" xlink:to="loc_us-gaap_SegmentDomain_44528d71-d681-4feb-aa8f-615544225e0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RiskandInsuranceServicesSegmentMember_f621f92f-8334-4cf7-a03c-afb373c72e2b" xlink:href="mmc-20230630.xsd#mmc_RiskandInsuranceServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_44528d71-d681-4feb-aa8f-615544225e0d" xlink:to="loc_mmc_RiskandInsuranceServicesSegmentMember_f621f92f-8334-4cf7-a03c-afb373c72e2b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ConsultingSegmentMember_2e3c572f-e2c5-4ce3-b140-21589a0983b3" xlink:href="mmc-20230630.xsd#mmc_ConsultingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_44528d71-d681-4feb-aa8f-615544225e0d" xlink:to="loc_mmc_ConsultingSegmentMember_2e3c572f-e2c5-4ce3-b140-21589a0983b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MercerUSAffinityBusinessMember_1ef3dc86-2085-42a4-b0b4-066698aafb00" xlink:href="mmc-20230630.xsd#mmc_MercerUSAffinityBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_44528d71-d681-4feb-aa8f-615544225e0d" xlink:to="loc_mmc_MercerUSAffinityBusinessMember_1ef3dc86-2085-42a4-b0b4-066698aafb00" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_SmallIndividualFinancialAdvisoryBusinessMember_55adcf1e-3341-4511-898a-900ec2b6e4d1" xlink:href="mmc-20230630.xsd#mmc_SmallIndividualFinancialAdvisoryBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_44528d71-d681-4feb-aa8f-615544225e0d" xlink:to="loc_mmc_SmallIndividualFinancialAdvisoryBusinessMember_55adcf1e-3341-4511-898a-900ec2b6e4d1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_OliverWymanGroupConsultingGroupMember_36acdbe3-6b04-49b7-902e-2bb4612fd5ed" xlink:href="mmc-20230630.xsd#mmc_OliverWymanGroupConsultingGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_44528d71-d681-4feb-aa8f-615544225e0d" xlink:to="loc_mmc_OliverWymanGroupConsultingGroupMember_36acdbe3-6b04-49b7-902e-2bb4612fd5ed" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://mmc.com/role/SegmentInformationDetailsofOperatingSegmentRevenueDetails" xlink:type="simple" xlink:href="mmc-20230630.xsd#SegmentInformationDetailsofOperatingSegmentRevenueDetails"/>
  <link:definitionLink xlink:role="http://mmc.com/role/SegmentInformationDetailsofOperatingSegmentRevenueDetails" xlink:type="extended" id="if6a66b2b48cb4992ab474af62ba824c6_SegmentInformationDetailsofOperatingSegmentRevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_41aa4897-cb09-48d8-8f12-ed0908435b18" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_6d890f8f-ad21-4bf6-93f8-74abb49e4e85" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_41aa4897-cb09-48d8-8f12-ed0908435b18" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_6d890f8f-ad21-4bf6-93f8-74abb49e4e85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_5c68ee0e-93e6-4033-ae9b-9a3cf8da9d06" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_41aa4897-cb09-48d8-8f12-ed0908435b18" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_5c68ee0e-93e6-4033-ae9b-9a3cf8da9d06" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_0e8307c4-4add-49cb-aed8-156b76c91881" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_5c68ee0e-93e6-4033-ae9b-9a3cf8da9d06" xlink:to="loc_srt_ConsolidationItemsAxis_0e8307c4-4add-49cb-aed8-156b76c91881" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_0e8307c4-4add-49cb-aed8-156b76c91881_default" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_0e8307c4-4add-49cb-aed8-156b76c91881" xlink:to="loc_srt_ConsolidationItemsDomain_0e8307c4-4add-49cb-aed8-156b76c91881_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_acbe54a4-fb5f-4c0d-a833-a7d127affc8f" xlink:href="https://xbrl.fasb.org/srt/2023/elts/srt-2023.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_0e8307c4-4add-49cb-aed8-156b76c91881" xlink:to="loc_srt_ConsolidationItemsDomain_acbe54a4-fb5f-4c0d-a833-a7d127affc8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_43444827-1d6b-435d-93d7-e097dedd2894" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_acbe54a4-fb5f-4c0d-a833-a7d127affc8f" xlink:to="loc_us-gaap_OperatingSegmentsMember_43444827-1d6b-435d-93d7-e097dedd2894" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_CorporateAndEliminationsMember_fa69a069-47fa-4981-855d-bf549a63b58f" xlink:href="mmc-20230630.xsd#mmc_CorporateAndEliminationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_acbe54a4-fb5f-4c0d-a833-a7d127affc8f" xlink:to="loc_mmc_CorporateAndEliminationsMember_fa69a069-47fa-4981-855d-bf549a63b58f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_df88dcd7-3a3b-4cf4-acd8-58f45151ca5e" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_5c68ee0e-93e6-4033-ae9b-9a3cf8da9d06" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_df88dcd7-3a3b-4cf4-acd8-58f45151ca5e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_df88dcd7-3a3b-4cf4-acd8-58f45151ca5e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_df88dcd7-3a3b-4cf4-acd8-58f45151ca5e" xlink:to="loc_us-gaap_SegmentDomain_df88dcd7-3a3b-4cf4-acd8-58f45151ca5e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_d72cbb84-0d16-4f05-a1ba-c0ba69f1bdef" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_df88dcd7-3a3b-4cf4-acd8-58f45151ca5e" xlink:to="loc_us-gaap_SegmentDomain_d72cbb84-0d16-4f05-a1ba-c0ba69f1bdef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_RiskandInsuranceServicesSegmentMember_78c4d339-8e73-41fc-a05d-af9914d68113" xlink:href="mmc-20230630.xsd#mmc_RiskandInsuranceServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_d72cbb84-0d16-4f05-a1ba-c0ba69f1bdef" xlink:to="loc_mmc_RiskandInsuranceServicesSegmentMember_78c4d339-8e73-41fc-a05d-af9914d68113" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_ConsultingSegmentMember_90806c86-f17f-48af-8ac3-ccee49f42075" xlink:href="mmc-20230630.xsd#mmc_ConsultingSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_d72cbb84-0d16-4f05-a1ba-c0ba69f1bdef" xlink:to="loc_mmc_ConsultingSegmentMember_90806c86-f17f-48af-8ac3-ccee49f42075" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsAxis_33cdbe4d-4b16-4274-ac55-b4f22db4ddf1" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_5c68ee0e-93e6-4033-ae9b-9a3cf8da9d06" xlink:to="loc_us-gaap_SubsegmentsAxis_33cdbe4d-4b16-4274-ac55-b4f22db4ddf1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_33cdbe4d-4b16-4274-ac55-b4f22db4ddf1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsegmentsAxis_33cdbe4d-4b16-4274-ac55-b4f22db4ddf1" xlink:to="loc_us-gaap_SubsegmentsDomain_33cdbe4d-4b16-4274-ac55-b4f22db4ddf1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsegmentsDomain_7800e4a0-e73d-436f-9ee4-3c18965ec61a" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_SubsegmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsegmentsAxis_33cdbe4d-4b16-4274-ac55-b4f22db4ddf1" xlink:to="loc_us-gaap_SubsegmentsDomain_7800e4a0-e73d-436f-9ee4-3c18965ec61a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MarshInsuranceGroupMember_b13821eb-36c8-41c6-b2a1-f5e2717b0595" xlink:href="mmc-20230630.xsd#mmc_MarshInsuranceGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_7800e4a0-e73d-436f-9ee4-3c18965ec61a" xlink:to="loc_mmc_MarshInsuranceGroupMember_b13821eb-36c8-41c6-b2a1-f5e2717b0595" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_GuyCarpenterReinsuranceGroupMember_82912699-9954-442a-abb3-a79d97f91e36" xlink:href="mmc-20230630.xsd#mmc_GuyCarpenterReinsuranceGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_7800e4a0-e73d-436f-9ee4-3c18965ec61a" xlink:to="loc_mmc_GuyCarpenterReinsuranceGroupMember_82912699-9954-442a-abb3-a79d97f91e36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_MercerConsultingGroupMember_b482c02d-2604-4b46-9837-07766553052e" xlink:href="mmc-20230630.xsd#mmc_MercerConsultingGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_7800e4a0-e73d-436f-9ee4-3c18965ec61a" xlink:to="loc_mmc_MercerConsultingGroupMember_b482c02d-2604-4b46-9837-07766553052e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mmc_OliverWymanGroupConsultingGroupMember_fe4f09de-2cdd-4570-9c30-f487bf3fc48a" xlink:href="mmc-20230630.xsd#mmc_OliverWymanGroupConsultingGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsegmentsDomain_7800e4a0-e73d-436f-9ee4-3c18965ec61a" xlink:to="loc_mmc_OliverWymanGroupConsultingGroupMember_fe4f09de-2cdd-4570-9c30-f487bf3fc48a" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
