The state highway runs 2.79 miles (4.49 km) from MD 51 in Evitts Creek north to Interstate 68 (I-68), U.S. Route 40 (US 40), US 220, and US 40 Alternate in Cumberland.

The state highway heads north as two-lane Messick Road, paralleling the east side of Evitts Creek.

After crossing the creek, MD 639 turns north onto Willowbrook Road, a two-lane undivided road that passes through two roundabouts at the entrance to the UPMC Western Maryland hospital and then enters the city limits of Cumberland.

After passing the entrance to Allegany College of Maryland, MD 639 turns northwest, passing Country Club Road before reaching its northern terminus at a diamond interchange with I-68, US 40, and US 220 (National Freeway).

The roadway continues as US 40 Alternate (Baltimore Avenue), which heads toward downtown Cumberland.