Loudoun County is a county located in the Commonwealth of Virginia. As of the 2010 census, the population was 312,311. making it the fourth-most populous county in Virginia, in 2013, the population was estimated to be 347,969, making it the third-most populous (surpassing Chesterfield County). Its county seat is Leesburg. As of 2007, the town had been county seat for 249 of the last 250 years.
Loudoun county is included in the Washington, D.C. metropolitan area.
, Loudoun County has the highest median household income of any county in the United States ($107,207), beating neighboring Fairfax County, Virginia ($105,241). The two counties have been trading places as the highest-income county in the United States in recent years.
Loudoun County was established in 1757 from Fairfax County. The county is named for John Campbell, Fourth Earl of Loudoun and Governor General of Virginia from 1756–59. Western settlement began in the 1720s and 1730s with Quakers, Scots-Irish, Germans and others moving south from Pennsylvania and Maryland and by English and African slaves moving upriver from Tidewater.
By the time of the American Revolution, it was the most populous county in Virginia. During the War of 1812, important Federal documents and government archives were evacuated from Washington and stored at Leesburg for safe keeping. Local tradition holds that these documents were stored at Rokeby House and thus that Leesburg was briefly the capital of the United States.
Early in the American Civil War, the Battle of Balls Bluff took place near Leesburg on October 21, 1861. Future jurist Oliver Wendell Holmes, Jr. was critically wounded in that battle along the Potomac River. During the Gettysburg Campaign in June 1863, Confederate Major General J.E.B. Stuart and Union cavalry clashed in the battles of Aldie, Middleburg, and Upperville. Confederate partisan John S. Mosby based his operations in Loudoun and adjoining Fauquier County (for a more in-depth account of the history of Loudoun County during the Civil War, see Loudoun County in the American Civil War).