| Cover | Aug. 05, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Aug. 05, 2022 |
| Current Fiscal Year End Date | --12-31 |
| Entity File Number | 001-41472 |
| Entity Registrant Name | MILL CITY VENTURES III, LTD. |
| Entity Central Index Key | 0001425355 |
| Entity Tax Identification Number | 90-0316651 |
| Entity Incorporation, State or Country Code | MN |
| Entity Address, Address Line One | 1907 Wayzata Boulevard |
| Entity Address, Address Line Two | Suite 205 |
| Entity Address, City or Town | Wayzata |
| Entity Address, State or Province | MN |
| Entity Address, Postal Zip Code | 55391 |
| City Area Code | 952 |
| Local Phone Number | 479-1923 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
