SR 9 is the main east–west highway of northwestern Loudoun County, connecting Leesburg with Hillsboro and the West Virginia cities of Charles Town and Martinsburg.

SR 9 heads southeast as a two-lane undivided road, passing to the west of Purcell Knob as the highway descends Blue Ridge Mountain to the village of Mechanicsville in a valley known as Between the Hills.

The state highway veers southeast and crosses the South Fork of Catoctin Creek before reaching the community of Paeonian Springs.

[3][4] There has been considerable debate about how to manage increasing traffic levels as the populations of both western Loudoun and Jefferson County, West Virginia have grown throughout the 21st century, turning what was a country road into a major commuter route.

[5] A significant physical obstacle is that the town completely fills Hillsboro Gap; Route 9 cannot be widened in-place, and any bypass would have to go over or tunnel through Short Hill Mountain, substantially increasing its cost.