SR 309 begins at an intersection with SR 123 (Dolley Madison Boulevard) in McLean. Old Dominion Drive continues northwest as SR 738 toward Great Falls. The state highway heads east as a four-lane divided highway through the center of McLean. East of Chain Bridge Road, SR 309 becomes a two-lane road and crosses Pimmit Run. The state highway passes through a densely populated residential area in northeastern Fairfax County and far western Arlington County. Shortly after entering the latter county, SR 309 expands to a four-lane divided street and intersects SR 120 (Glebe Road) at an oblique intersection adjacent to Marymount University; at the south end of the intersection, the two highways cross over Yorktown Boulevard. At 26th Street, which connects the highway with Yorktown Boulevard and the university, the state highway becomes two lanes again. SR 309 briefly runs concurrently with US 29 as four-lane divided Old Dominion Drive. At the highways' western junction, at which there is no direct access from northbound US 29 to westbound SR 309, the U.S. Highway heads toward Falls Church as Lee Highway. The two highways diverge, with US 29 following Old Dominion Drive and SR 309 using the parallel two-lane Lee Highway. Within Arlington's Cherrydale neighborhood, SR 309 reaches its eastern terminus at a five-way intersection with US 29, Quincy Street, and Military Road. Lee Highway continues east along US 29 toward Rosslyn.