The state highway is known as Seminary Road and Janneys Lane on its 2.75-mile (4.43 km) course from Interstate 395 (I-395) east to SR 7 within the independent city of Alexandria.

SR 420 begins at a three-level diamond interchange with I-395 (Henry G. Shirley Memorial Highway).

The roadway continues west as an unnumbered segment of Seminary Road that passes by the Mark Center Building, a massive office building for the United States Department of Defense, on its way to Bailey's Crossroads.

SR 420 reduces to a four-lane undivided street that passes by Inova Alexandria Hospital, after which it again narrows to a two-lane road with a center left-turn lane.

The state highway passes its namesake, the Virginia Theological Seminary, just west of its intersection with Quaker Lane, which heads north as SR 402.