PA 851 is a two-lane undivided road that runs through rural areas in southern York County a short distance to the north of the Maryland border. The route heads east from Sticks to New Freedom, where it turns north and intersects PA 616 in Railroad. PA 851 heads east through Shrewsbury and comes to an interchange with Interstate 83 (I-83). From here, the route heads east and passes through Stewartstown, where it forms a short concurrency with PA 24, and Fawn Grove, where it intersects PA 425, before continuing to its terminus. PA 851 has a truck route, PA 851 Truck, which bypasses the stretch between New Freedom and Shrewsbury.
