Loudoun County is in the northern part of the Commonwealth of Virginia in the United States. In 2020, the census returned a population of 420,959, making it Virginia's third-most populous county. The county seat is Leesburg. Loudoun County is part of the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area.

Oak Hill is a mansion and plantation located in Aldie, Virginia that was for 22 years a home of Founding Father James Monroe, the fifth U.S. President. It is located approximately 9 miles (14 km) south of Leesburg on U.S. Route 15, in an unincorporated area of Loudoun County, Virginia. Its entrance is 10,300 feet (3,100 m) north of Gilberts Corner, the intersection of 15 with U.S. Route 50. It is a National Historic Landmark, but privately owned and not open to the public.