SR 120 begins at an intersection with US 1 (Jefferson Davis Highway) at the south end of Crystal City, just north of the city of Alexandria––though Glebe Road actually continues three blocks east of US 1 where it ends at Potomac Avenue. The state highway heads west as a four-lane divided highway through the Arlington County sewage treatment facility. West of the facility, SR 120 parallels the Four Mile Run Trail and Four Mile Run, both to the south, and passes to the south of the Arlington Ridge community. 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 its oblique intersection with Walter Reed Drive, the state highway passes through the commercial center of Westmont, where the highway intersects SR 244 (Columbia Pike). North of Westmont, SR 120 has a partial cloverleaf interchange with US 50 (Arlington Boulevard) adjacent to the Cathedral of Saint Thomas More.