SR 120 leaves Ballston as a four-lane divided highway that passes by The Glebe House and intersects US 29 (Lee Highway). The state highway has an oblique intersection with SR 309 (Old Dominion Drive) on an overpass of Yorktown Boulevard adjacent to Marymount University. At Williamsburg Boulevard, SR 120 veers northeast to parallel the Arlington–Fairfax county line. At the top of its descent to the Potomac River, the state highway has a partial cloverleaf interchange with Military Road, a residential street, and reduces to a two-lane undivided road. The highway has a sweeping curve to the north and passes below the George Washington Memorial Parkway immediately before its northern terminus at SR 123 (Chain Bridge Road), which connects SR 120 with the Parkway. The road continues straight as SR 123 to the Chain Bridge, which crosses the Potomac River just below Little Falls to an intersection with Canal Road and the Clara Barton Parkway in Washington a short distance east of the District of Columbia–Maryland boundary.