v3.22.0.1
Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2021
Mar. 15, 2022
Jun. 30, 2021
Document and Entity Information      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 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 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 false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Public Float     $ 20,272,879
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 FY    
Amendment Flag false    
No Trading Symbol Flag true    
Auditor Name Boulay PLLP    
Auditor Firm ID 542    
Auditor Location Minneapolis, Minnesota