PA 642 begins at a signalized intersection where Third Street (designated SR 1101 by the
Pennsylvania Department of Transportation (PennDOT)) and Broad Street (SR 1004) in the community of West Milton in
Kelly Township,
Union County. The intersection is located just east of an interchange with the US 15 freeway. Third Street is a former section of US 15 and is used to access the northbound lanes of the freeway. PA 642 heads east on two-lane undivided Broad Street. The road passes through residential areas with some businesses before crossing a
Union County Industrial Railroad line. The route crosses the
West Branch Susquehanna River into the borough of
Milton in
Northumberland County, where the road becomes Mahoning Street and heads onto Montgomery Island, which contains
Milton State Park. On the island, the road briefly becomes a
divided highway and comes to a
right-in/right-out access point to the state park. PA 642 crosses to the east bank of the river and heads southeast through commercial areas, intersecting a
one-way pair that carries
PA 405. The road crosses
Norfolk Southern's
Buffalo Line and heads through residential areas. The route enters
Turbot Township and runs past more homes, coming to an interchange with the
PA 147 freeway. Past this interchange, PA 642 crosses into
East Chillisquaque Township and becomes an unnamed road, heading through agricultural areas with some woods and homes. The road turns east and heads through Pottsgrove before crossing the
Chillisquaque Creek. PA 642 continues into
Liberty Township in
Montour County and becomes Liberty Valley Road, running through more open agricultural areas with some residences. The road heads into woodland and intersects the eastern terminus of
PA 45, continuing through a mix of farmland and woods with homes. The route heads through more rural areas, crossing into
Valley Township and passing through the residential community of
Mausdale before intersecting
PA 54. A
park and ride lot is located east of this intersection. Here, PA 642 turns southeast to form a
concurrency with PA 54 on Continental Boulevard, a four-lane divided highway. PA 642 splits from PA 54 by heading northeast on two-lane undivided Jerseytown Road. The road heads through farmland and woodland with some homes, passing over
I-80. The route continues north into more forested areas with some fields and residences, becoming the border between
Derry Township to the west and
West Hemlock Township to the east. PA 642 turns northeast to fully enter West Hemlock Township and runs through more agricultural and wooded areas with occasional homes. ==History==