Boolean flag that is true when the XBRL content amends previously-filed or accepted submission.

No definition available.

Name:	dei_AmendmentFlag
Namespace Prefix:	dei_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

PCAOB issued Audit Firm Identifier

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 10-K
-Number 249
-Section 310

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 20-F
-Number 249
-Section 220
-Subsection f

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form 40-F
-Number 249
-Section 240
-Subsection f

Document and Entity Information - USD ($) $ in Thousands	12 Months Ended
Document and Entity Information - USD ($) $ in Thousands	Dec. 31, 2021	Mar. 14, 2022	Jun. 30, 2021
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Registrant Name	MainStreet Bancshares, Inc.
Entity Central Index Key	0001693577
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
ICFR Auditor Attestation Flag	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		7,648,973
Entity Public Float			$ 170,465,407
Entity File Number	001-38817
Entity Incorporation, State or Country Code	VA
Entity Tax Identification Number	81-2871064
Entity Address, Address Line One	10089 Fairfax Boulevard
Entity Address, City or Town	Fairfax
Entity Address, State or Province	VA
Entity Address, Postal Zip Code	22030
City Area Code	703
Local Phone Number	481-4567
Document Annual Report	true
Document Transition Report	false
Auditor Firm ID	613
Auditor Name	Yount, Hyde & Barbour, P.C.
Auditor Location	Winchester, Virginia, USA
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE: The information required by Part III of this Annual Report on Form 10-K will be found in the Company’s definitive proxy statement for its 2021 Annual Meeting of Shareholders, to be filed pursuant to Regulation 14A under the Securities Exchange Act of 1934, and such information is incorporated herein by this reference.
Common Stock [Member]
Document Information [Line Items]
Trading Symbol	MNSB
Title of 12(b) Security	Common Stock
Security Exchange Name	NASDAQ
Depositary Shares [Member]
Document Information [Line Items]
Trading Symbol	MNSBP
Title of 12(b) Security	Depositary Shares (each representing a 1/40th interest in a share of 7.50% Series A Fixed-Rate Non-Cumulative Perpetual Preferred Stock)
Security Exchange Name	NASDAQ