SR 9 begins at the West Virginia state line at Keyes Gap, a wind gap on top of Blue Ridge Mountain. The highway continues west as WV 9 to Charles Town. SR 9 heads southeast as a two-lane 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. At the eastern edge of the narrow north-south valley, the state highway parallels the North Fork of Catoctin Creek through Hillsboro Gap, a water gap in Short Hill Mountain, into the town of Hillsboro. SR 9 continues east through the much wider Catoctin Valley, crossing the North Fork and intersecting SR 287 in the hamlet of Wheatland. The state highway veers southeast and crosses the South Fork of Catoctin Creek before reaching the community of Paeonian Springs. SR 9 curves south to its southern terminus at Clarke's Gap, a low point in Catoctin Mountain. The state highway expands to a four-lane divided highway and meets SR 7 (Harry Byrd Highway) at a diamond interchange. SR 9 is paralleled through the interchange by the Washington and Old Dominion Trail. At the southern end of the interchange, the highway continues southward as SR 7 Business (Colonial Highway), which immediately turns west toward Hamilton.