PA 103's southern terminus is an at-grade intersection with US 522 at Allenport in Shirley Township, a bit south of US 522's bridge across the Juniata River in Mount Union.

PA 103 begins at an intersection with US 522 in the community of Allenport in Shirley Township, Huntingdon County, heading east on two-lane undivided Beacon Lodge Road.

The road immediately crosses the East Broad Top Railroad before it curves southeast and runs between residential areas to the southwest and the Juniata River to the northeast.

[2][3] PA 103 enters Wayne Township in Mifflin County and becomes an unnamed road, curving to the northwest and west as it continues parallel to the Juniata River through forests.

PA 103 crosses into Bratton Township and winds northeast through farmland with some woods and homes to the southeast of the Juniata River and Norfolk Southern's Pittsburgh Line.

[2][4] PA 103 reaches the residential community of Mattawana, where it passes through two S-curves to the northwest before making a turn to the east at the intersection with John Street, which heads west across the Juniata River to the borough of McVeytown.

The route enters Granville Township and the river and railroad line head further away from the road as it passes through a mix of farmland and woodland with some homes a short distance northwest of Blue Mountain.

The route turns north and leaves Juniata Terrace for Granville Township again as it comes to a bridge over the Pittsburgh Line a short distance east of Lewistown's Amtrak station.