PA 851 begins at an intersection with
PA 516 just south of
PA 216 in the community of Sticks in
Codorus Township,
York County, heading southeast on Steltz Road. The road winds southeast through farmland with some woods and homes, eventually bending to the south. The route turns east to remain along Steltz Road and runs a short distance to the north of the
Maryland border, passing through the community of Steitz and crossing into
Shrewsbury Township. PA 851 turns northeast away from the state line and continues through a mix of farm fields and woodland with occasional residences. The road curves east and then southeast before it makes a sharp turn to the northeast. PA 851 enters the borough of
Shrewsbury and becomes West Railroad Avenue, passing residences with some farmland to the south. The route turns north onto Park Avenue before heading east along West Forrest Avenue. PA 851 runs past homes and businesses in the center of Shrewsbury and crosses Main Street, where PA 851 Truck returns to the route. Past this intersection, the road becomes East Forrest Avenue and runs past more residences before gaining a
center left-turn lane and heading through business areas along the border between Shrewsbury to the north and Shrewsbury Township to the south. The route fully enters Shrewsbury Township and comes to a
diverging diamond interchange with
I-83. Following this interchange, PA 851 runs past more commercial establishments before narrowing to two lanes and heading southeast, passing to the north of a
park and ride lot. The route heads into farmland with some woods and homes parallel to
Deer Creek to the northeast. The road curves east and crosses Deer Creek into
Hopewell Township, continuing through rural areas with some development as Bridgeview Road. The route passes through the communities of Orwig, Reimold, and Zeigler before it runs through residential areas and heads into the borough of
Stewartstown. Here, PA 851 becomes North Hill Street and heads southeast through industrial and commercial areas, crossing the
Stewartstown Railroad. The route passes homes and some businesses, turning east onto West Pennsylvania Avenue and continuing to an intersection with
PA 24 in the center of Stewartstown. At this point, PA 851 turns south for a
concurrency with PA 24 on North Main Street for a block. PA 851 splits from PA 24 by heading east onto College Avenue, heading through residential areas. The route leaves Stewartstown for Hopewell Township again and becomes Pleasant Valley Road, running through agricultural areas with occasional trees and homes. PA 851 curves northeast before it turns southeast onto Woolen Mill Road and winds through a mix of farmland and woodland with some homes. The road crosses the South Branch
Muddy Creek into
Fawn Township and continues through rural areas, curving east to pass through residential areas in the community of
New Park. The route becomes West Main Street and runs through farmland with some trees and development, bending southeast as it passes south of
Kennard-Dale High School and entering the borough of
Fawn Grove. PA 851 runs between homes to the north and farm fields to the south before running past residences and businesses in the center of the borough, where it intersects the southern terminus of
PA 425. At this point, the road becomes East Main Street and heads east past more homes before passing through a mix of residences and farmland. The route heads back into Fawn Township and becomes Graceton Road, heading southeast through agricultural areas with some woods and homes. The road curves east and runs immediately to the north of the Maryland border. In the community of Constitution, PA 851 turns north away from the state line onto Bryansville Road, while Graceton Road continues along the state line as State Route 2080 (SR 2080), an unsigned
quadrant route, before curving into Maryland and becoming
Maryland Route 624. PA 851 heads through farmland with some woods and homes and bends northeast, crossing into
Peach Bottom Township. The road continues through rural land and turns to the east at the Five Forks intersection with Miller Road, Hushon Road, and Aubel Road, passing to the south of the residential development of
Susquehanna Trails. The route winds east through forests before running through farm fields with some homes and passing through the community of
Bryansville. The road winds northeast through forests, turning southeast in the community of Bryansville Station. PA 851 continues through fields with some development before it comes to its eastern terminus at a
roundabout with
PA 74, where SR 2045 (Broad Street) continues southeast towards the borough of
Delta. ==History==