A median was installed in 1963 from Minersville to US 209.

In 1971, the western terminus was moved to its current location.  Prior to that, it was located at US 209 and formed a loop into Minersville.  In 1987, the eastern terminus was moved to its current location from US 209.

In 1940, the route between Milesville and Monessen Elizabeth Road was paved.  Paved in 1946 from Milesville to Sunnyside Road.

In 1973, the northern terminus was moved from PA 51 at Monessen Elizabeth Road to its current location.

The section between Craighton Run Road and Russellton Road was paved in 1940.  In 1941, the section between Russellton Road and Millerstown was paved.

The section from Mill Dam Road to Dorseyville was improved and the section between the West Deer Township line and Dorseyville was paved in 1940.  In 1946, that latter section was paved.

In 1977, the western terminus was moved to its current terminus from PA 8.

On July 25, 2003, the section from Mountain View Road to Campbell Road was closed when it began to sink.  It reopened on August 28, 2003.

In 1946, the section from Akersville to one-half mile north was improved.  In 1947, the section from Akersville to US 30 was paved.

In 1964, the southern terminus was moved from US 30 to its current location.

Signed in 1964 on its current alignment.  The northern terminus moved from I-70 to its current location in 1965.

The southern terminus was moved from Shenandoah Heights to Frackville in 1946.

A median was installed in 1957 from Gilberton to Shenandoah.

In 1965, the route was divded between Frackville to Gilberton and a year later from Humboldt to Hazleton.

In October 2003, PennDOT began construction of a new bridge to replace the "singing bridge" at the intersection with Westtown Road.  The reason it was called the the "singing bridge" is from the "whirrr" noise produced by tires as vehicles crossed.  The new structure cost $790,000 to build, which was just for deck replacement as the bridge foundation is still intact.

Signed in 1936 on the current PA 51 alignment.

In 1946, the route's southern terminus was moved from PA 317 in Bessemer to PA 208 in New Bedford.  Also the route was paved between US 224 and New Castle and Struthers Road, and from the Lawrence County line to PA 318.

In 1973, the southern terminus was moved to US 422 from PA 208 in New Bedford.

In 1970, the northern terminus was moved from Lickdale to PA 443 just north of I-81.  In 1976, the northern terminus was moved to its current location.

In 1934, the section from East Side to Blakeslee Corners was under construction and opened the next year.  In 1935, the eastern terminus was moved from Blakeslee Corners to Swiftwater.

In 1964, the eastern terminus was moved from Swiftwater to its current location.  Also in that year, the alignment from I-380 to PA 314 was upgraded to an expressway, and a median was installed from PA 314 to PA 191.