| Document and Entity Information | Apr. 26, 2022 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Apr. 26, 2022 |
| Entity File Number | 001-41315 |
| Entity Registrant Name | John Marshall Bancorp, Inc. |
| Entity Incorporation, State or Country Code | VA |
| Entity Tax Identification Number | 81-5424879 |
| Entity Address State Or Province | VA |
| Entity Address, Address Line One | 1943 Isaac Newton Square |
| Entity Address, Adress Line Two | Suite 100 |
| Entity Address, City or Town | Reston |
| Entity Address, Postal Zip Code | 20190 |
| City Area Code | 703 |
| Local Phone Number | 584-0840 |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | JMSB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity Central Index Key | 0001710482 |
| Amendment Flag | false |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
