PA 125 crosses into Upper Mahanoy Township in Northumberland County and becomes an unnamed road, soon turning to the north in Line Mountain. The road winds through a mix of farmland and woodland before turning northwest to ascend forested Line Mountain. The route heads into East Cameron Township and turns northeast to descend the mountain. PA 125 crosses the Mahanoy Creek and winds northeast through a valley, passing through areas of farmland and woodland. The road passes through Gowen City and turns northwest to cross forested Mahanoy Mountain, crossing into Coal Township. The route passes a few fields and runs through Burnside, heading into more forested areas. PA 125 curves more to the north and becomes Bear Valley Road, curving northeast and heading into the city of Shamokin. At this point, the road becomes Bear Valley Avenue and heads into residential areas. PA 125 turns to the north and becomes South Market Street, gaining a wide median as it passes more homes. The route heads into the commercial downtown and becomes North Market Street, crossing the Shamokin Valley Railroad and passing more businesses. PA 125 becomes an undivided road again for a block before ending at PA 61.