Known as Glebe Road, the state highway runs 9.10 miles (14.65 km) from U.S. Route 1 (US 1) in Crystal City north to SR 123 at the Chain Bridge.

At the point the old alignment of SR 120, also named Glebe Road, crosses Four Mile Run into Alexandria, the state highway veers northwest away from the stream and meets I-395 (Henry G. Shirley Memorial Highway) at a partial cloverleaf interchange north of Shirlington.

North of the freeway, SR 120 passes between the historic Lomax African Methodist Episcopal Zion Church and Army Navy Country Club.

North of Westmont, SR 120 has a partial cloverleaf interchange with US 50 (Arlington Boulevard) adjacent to the Cathedral of Saint Thomas More.

[1][3] SR 120 continues north onto Ballston, where the highway expands to six lanes at Quincy Street.

The state highway has an oblique intersection with SR 309 (Old Dominion Drive) on an overpass of Yorktown Boulevard adjacent to Marymount University.