<?xml version="1.0" encoding="utf-8"?>
<!--Generated by DFIN XBRL Instance Document - http://www.dfinsolutions.com/ - Version R3.0 - on 27-March-2023 [06:56:12] {PM}-->
<schema targetNamespace="http://mmc.com/20221231" elementFormDefault="qualified" xmlns:mmc="http://mmc.com/20221231" xmlns="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xl="http://www.xbrl.org/2003/XLink" xmlns:us-gaap="http://fasb.org/us-gaap/2022" xmlns:country="http://xbrl.sec.gov/country/2022" xmlns:dei="http://xbrl.sec.gov/dei/2022q4" xmlns:us-types="http://fasb.org/us-types/2022" xmlns:deprecated="http://www.xbrl.org/2009/arcrole/fact-explanatoryFact" xmlns:srt="http://fasb.org/srt/2022" xmlns:srt-types="http://fasb.org/srt-types/2022" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2020-01-21">
<annotation>
<appinfo>
<link:linkbaseRef xlink:type="simple" xlink:href="mmc-20221231_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" />
<link:linkbaseRef xlink:type="simple" xlink:href="mmc-20221231_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" />
<link:linkbaseRef xlink:type="simple" xlink:href="mmc-20221231_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" />
</appinfo>
</annotation>
<import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" />
<import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" />
<import namespace="http://xbrl.sec.gov/dei/2022q4" schemaLocation="https://xbrl.sec.gov/dei/2022q4/dei-2022q4.xsd" />
<import namespace="http://xbrl.sec.gov/dei-sub/2022q4" schemaLocation="https://xbrl.sec.gov/dei/2022q4/dei-sub-2022q4.xsd" />
<import namespace="http://www.xbrl.org/dtr/type/2020-01-21" schemaLocation="https://www.xbrl.org/dtr/type/2020-01-21/types.xsd" />
<import namespace="http://fasb.org/srt/2022" schemaLocation="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd" />
<import namespace="http://fasb.org/srt-types/2022" schemaLocation="https://xbrl.fasb.org/srt/2022/elts/srt-types-2022.xsd" />
<import namespace="http://xbrl.sec.gov/ecd-sub/2022q4" schemaLocation="https://xbrl.sec.gov/ecd/2022q4/ecd-sub-2022q4.xsd" />
<import namespace="http://xbrl.sec.gov/ecd/2022q4" schemaLocation="https://xbrl.sec.gov/ecd/2022q4/ecd-2022q4.xsd" />
<import namespace="http://www.xbrl.org/2006/ref" schemaLocation="http://www.xbrl.org/2006/ref-2006-02-27.xsd" />
<import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" />
<import namespace="http://fasb.org/srt-roles/2022" schemaLocation="https://xbrl.fasb.org/srt/2022/elts/srt-roles-2022.xsd" />
<element name="FairValueOfEquityAwardsGrantedDuringFiscalYearReportedInSctMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_FairValueOfEquityAwardsGrantedDuringFiscalYearReportedInSctMember" substitutionGroup="xbrli:item" />
<element name="ChangeInPensionValueReportedInSctMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_ChangeInPensionValueReportedInSctMember" substitutionGroup="xbrli:item" />
<element name="AdjustmentsToPeoSctTotalMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_AdjustmentsToPeoSctTotalMember" substitutionGroup="xbrli:item" />
<element name="YearEndFairValueOfEquityAwardsGrantedDuringTheFiscalYearThatAreOutstandingAndUnvestedMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_YearEndFairValueOfEquityAwardsGrantedDuringTheFiscalYearThatAreOutstandingAndUnvestedMember" substitutionGroup="xbrli:item" />
<element name="ChangeInFairValueOfPriorYearAwardsThatAreOutstandingAndUnvestedMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_ChangeInFairValueOfPriorYearAwardsThatAreOutstandingAndUnvestedMember" substitutionGroup="xbrli:item" />
<element name="ChangeInFairValueFromPriorYearendOfPriorYearAwardsThatVestedDuringTheFiscalYearMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_ChangeInFairValueFromPriorYearendOfPriorYearAwardsThatVestedDuringTheFiscalYearMember" substitutionGroup="xbrli:item" />
<element name="DividendsPaidOnStockOrStockOptionAwardsNotOtherwiseReflectedInFairValueOfTotalCompensationMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_DividendsPaidOnStockOrStockOptionAwardsNotOtherwiseReflectedInFairValueOfTotalCompensationMember" substitutionGroup="xbrli:item" />
<element name="TotalAdjustmentsMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="mmc_TotalAdjustmentsMember" substitutionGroup="xbrli:item" />
</schema>
