Loudoun County’s flag was authorized by the College of Arms in London and adopted by the Board of Supervisors in 1968. The flag is based on the Coat of Arms of the Fourth Earl of Loudoun, who was once Commander-in-Chief of the British Armed Forces in North America and Governor of Virginia.

The flag’s design represents the agricultural background of the county (the green color); the Monroe Doctrine and President Monroe’s association with the county (the indented or “embattled” edging); and the dairy industry (the white or silver drops suggestive of milk). Loudoun is believed to be the first county in the United States to have a flag based on a coat of arms that was officially approved by London's College of Arms.