MD 550 heads northwest out of the town of Thurmont and passes Catoctin High School as Sabillasville Road. The state highway follows the northern boundary of Catoctin Mountain Park as it starts to wind its way through the narrow valley of Owens Creek between Piney Mountain to the north and Catoctin Mountain to the south, passing under the parallel Maryland Midland Railway line multiple times. MD 550 leaves Owens Creek in the hamlet of Lantz and passes through the Harbaugh Valley, where the highway crosses Middle Creek, crosses under the railroad, and passes through the village of Sabillasville. The state highway crosses over Friends Creek then leaves the valley and ascends the eastern flank of South Mountain. MD 550 passes the Victor Cullen School Power House and the road to the preserved Victor Cullen Center before curving to the west at the top of the mountain, passing within of the Pennsylvania state line as the highway enters Washington County just south of Blue Ridge Summit.