In the 1930s, the southern terminus was realigned to US 22/PA 12 at Center Street and Elizabeth Avenue in Bethlehem, with PA 946 replacing the route between Hecktown and Moorestown.

PA 512 begins at an interchange with the US 22 freeway in Hanover Township in Northampton County, which is in the Lehigh Valley, heading north on four-lane divided Bath Pike.

The road heads into East Allen Township and narrows to two lanes, continuing through agricultural areas with some homes as Beth-Bath Pike.

PA 512 crosses the Monocacy Creek and Norfolk Southern's Cement Secondary, passing through Clyde.

Farther north, the route runs through commercial areas to the east of a cement plant before it enters the borough of Bath upon crossing the Norfolk Southern railroad tracks again.

The road narrows to two lanes and runs through a mix of farms, woods, and residences, passing to the east of a golf course and serving Chapman Quarries.

The route curves northeast and continues through rural areas with some residential subdivisions before it reaches an intersection with PA 946 in Moorestown.

The road continues northeast through a mix of farmland and woodland with a few homes, crossing Bushkill Creek and passing through Clearfield.

Past this interchange, PA 512 narrows to two lanes and intersects Sullivan Trail, at which point it turns north and enters the borough of Wind Gap.

PA 512 turns north onto East Main Street and curves northeast past a mix of homes and businesses.

PA 512 heads northeast along Bill Scott Boulevard through forested areas with some developments, running along the east side of the railroad tracks.

[5] When the U.S. Highway System was established in 1926, the section of paved road between Bangor and Mount Bethel was designated as part of US 611, which ran concurrent with PA 2.

[10][12] Also, the portion of road between Bangor and Mount Bethel became the northern segment of PA 712 following the rerouting of US 611 to a different alignment to the east.