Document and Entity Information - USD ($)	12 Months Ended
	Jun. 30, 2022	Sep. 27, 2022	Dec. 31, 2021
Document and Entity Information [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Jun. 30, 2022
Document Transition Report	false
Entity File Number	000-56269
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	None
Trading Symbol	None
Security Exchange Name	NONE
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
Entity Shell Company	false
Entity Public Float			$ 15,250,144
Entity Common Stock, Shares Outstanding		2,269,700
Entity Central Index Key	0001835385
Current Fiscal Year End Date	--06-30
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	FY
Amendment Flag	false
Auditor Name	Bonadio & Co., LLP
Auditor Firm ID	1884
Auditor Location	Syracuse, New York