MD 27 begins as Ridge Road at an intersection with MD 355 (Frederick Road) on the northern edge of Germantown. Ridge Road continues west as a six-lane divided county highway to a partial cloverleaf interchange with I-270 (Eisenhower Memorial Highway), beyond which the county highway continues as Father Hurley Boulevard, one of the primary arteries of Germantown. MD 27 curves north through a directional intersection with Henderson Corner Road and reduces from a six-lane divided highway to a two-lane undivided road past its intersection with Brink Road. The highway passes through the hamlet of Cedar Grove on its way to Damascus. At Damascus High School, MD 27 gains a center left-turn lane, a second lane northbound, then becomes a four-lane undivided highway. The highway becomes two lanes again at the west end of downtown Damascus, curves east, then turns north to remain on Ridge Road. The road continues straight east as MD 108 (Main Street), which leads to MD 124 (Woodfield Road). MD 27 starts out of Damascus as a four-lane undivided highway but quickly drops to two lanes. At the north end of the unincorporated town, the state highway meets the northern end of county-maintained Woodfield Road. MD 27 meets the eastern end of MD 80 (Kemptown Road) in the hamlet of Claggettsville. The highway crosses the Patuxent River just south of its source and passes through the western edge of Howard County. Between a pair of intersections with Penn Shop Road, MD 27 crosses a sliver of Montgomery County before entering Frederick County and expanding to a four-lane divided highway. The highway meets the west end of the Mount Airy–Ellicott City segment of MD 144 (Old National Pike). The intersection is just west of Parrs Spring, the source of the South Branch of the Patapsco River. Parrs Spring contains the quadripoint where the boundaries of Montgomery, Howard, Frederick, and Carroll counties meet. North of MD 144, MD 27 briefly follows the Frederick–Carroll county line before fully entering Carroll County and the town of Mount Airy at its partial cloverleaf interchange with I-70 and US 40 (Baltimore National Pike). The west leg of MD 27's intersection with the ramp from westbound I-70 meets the southern end of Main Street, which becomes MD 808 north of Ridgeville Boulevard. The MD 27–Main Street intersection is directly above the tunnel CSX's Old Main Line Subdivision uses to passes through Parrs Ridge.