Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2023	Nov. 03, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2023
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, Address Line One	1943 Isaac Newton Square East
Entity Address, Address Line Two	Suite 100
Entity Address, City or Town	Reston
Entity Address State Or Province	VA
Entity Address, Postal Zip Code	20190
City Area Code	703
Local Phone Number	584-0840
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	JMSB
Security Exchange Name	NASDAQ
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
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		14,126,084
Entity Central Index Key	0001710482
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Amendment Flag	false