Settling of the Loudoun area began between 1725 and 1730, while it was still owned by Lord Fairfax. Permanent settlers came from Pennsylvania, New Jersey and Maryland. During the same period, settlers from eastern Virginia of English Cavalier stock came to lower Loudoun and established large tobacco plantations.
In 1757, by act of the Virginia House of Burgesses, Fairfax County was divided. The western portion was named Loudoun for John Campbell, Fourth Earl of Loudoun, a Scottish nobleman who served as Commander-in-Chief for all British armed forces in North America and titular Governor of Virginia from 1756 to 1768.
Source History of Loudoun County, Virginia
Fairfax, Prince William, Fauquier and Clarke counties as well as Jefferson County, West Virginia. Across the Potomac River in Maryland, other adjacent counties are Washington, Frederick and Montgomery.