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Document and Entity Information - USD ($)
 12 Months Ended
Jun. 30, 2025
Sep. 25, 2025
Dec. 31, 2024
Document and Entity Information [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Jun. 30, 2025    
Document Transition Report false    
Entity File Number 001-42608    
Entity Registrant Name MARATHON BANCORP, INC.    
Entity Incorporation, State or Country Code MD    
Entity Tax Identification Number 86-2191258    
Entity Address, Address Line One 500 Scott Street    
Entity Address, City or Town Wausau    
Entity Address State Or Province WI    
Entity Address, Postal Zip Code 54403    
City Area Code 715    
Local Phone Number 845-7331    
Title of 12(b) Security Common Stock, $.01 par value    
No Trading Symbol Flag true    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
ICFR Auditor Attestation Flag false    
Entity Ex Transition Period false    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 20,911,632
Entity Common Stock, Shares Outstanding   2,938,698  
Documents Incorporated by Reference

DOCUMENTS INCORPORATED BY REFERENCE:

Proxy Statement for the 2025 Annual Meeting of Stockholders of the Registrant (Part III).

    
Entity Central Index Key 0001835385    
Current Fiscal Year End Date --06-30    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Amendment Flag false    
Auditor Name Bonadio & Co., LLP    
Auditor Location Pittsford, New York    
Auditor Firm ID 1884