Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 12, 2021
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2021
Entity File Number	814-00991
Entity Registrant Name	MILL CITY VENTURES III, LTD.
Entity Incorporation, State or Country Code	MN
Entity Tax Identification Number	90-0316651
Entity Address State Or Province	MN
Entity Address, Address Line One	1907 Wayzata Blvd
Entity Address, Address Line 2	#205
Entity Address, City or Town	Wayzata
Entity Address, Postal Zip Code	55391
City Area Code	952
Local Phone Number	479-1923
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		10,790,413
Entity Central Index Key	0001425355
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false
No Trading Symbol Flag	true