PA 551 enters the borough of Enon Valley in Lawrence County and becomes Main Street, heading into residential areas and crossing Norfolk Southern's Fort Wayne Line. The route intersects PA 351 and becomes concurrent with that route, soon heading north onto State Street as it passes more homes. The road heads into Little Beaver Township and becomes unnamed, heading through more areas of farms, woods, and residences as it passes through Old Enon. PA 351 splits to the northwest and PA 551 continues north on Enon Road, crossing the North Fork Little Beaver Creek again and continuing through more farmland and woodland with a few homes. The road heads into open agricultural areas with occasional residences, passing over I-76 (Pennsylvania Turnpike). The route crosses into North Beaver Township and runs through Newburg, continuing through more rural areas. PA 551 heads through more farmland with some woods and homes as it comes to a junction with PA 108. At this point, PA 108 turns north to form a concurrency with PA 551, with the name changing to Mt. Jackson Road. The road curves northeast through more agricultural areas with some woodland and residences, turning more to the east into wooded areas with some homes. In the community of Mount Jackson, PA 551 splits from PA 108 by heading north along with PA 317 on Mohawk School Road. The road passes farms and homes before PA 551 splits from PA 317 by heading north on Edinburg Road.