MD 77 begins at an intersection with MD 64 (Smithsburg Pike) in Smithsburg. The roadway continues northwest as county-maintained Leitersburg Smithsburg Road into the town of Smithsburg, where it becomes that town's Main Street, then continues northwest to Leitersburg. MD 77's first intersection east of MD 64 is Wolfsville Road, which becomes MD 17 at the Washington-Frederick county line and heads into the Middletown Valley. MD 77 heads east as Foxville Road, a two-lane undivided road that passes through several sharp curves as the highway ascends South Mountain. The state highway passes Pleasant Valley Road before it reaches its highest point at the Washington-Frederick County line. East of Foxville, MD 77 begins to follows the boundary between Cunningham Falls State Park to the south and the National Park Service-managed Catoctin Mountain Park to the north. Catoctin Mountain Park contains three historic camps: Camp Greentop, Camp Misty Mount, and the Presidential retreat Camp David.