In the late 18th century, following the American Revolution, this area of Fairfax County began to be referred to as Union Mills. Covering five or six square miles between Popes Head Run and Johnny Moore Run, it was recognized for its water powered mills. In addition to milling, local industries included talc quarrying, farming, grazing and lumbering.
During the Civil War, Union Mills became strategically important to the Confederates due to the routing of the Orange and Alexandria Railroad through the region and as a defensive position for Manassas Junction. Ironically, Union Mills' location, which should have assured its success, led to its decline as nearby Clifton developed into a railroad and lumbering center.
Overgrown mill races, the quarry outcropping and the remaining Confederate fortifications, which are listed on the National Register of Historic Places, are all reminders of Union Mills history.
Union Mills is a Fairfax County Historic Site.