Though formerly quite extensive, Lower St. Clair has become one of the smallest subdivisions of the county. It originally extended from Chartiers creek to Street’s run, including all that part of Pittsburgh on the South Side, the boroughs of Knoxville, Beltzhoover, West Liberty and Green Tree, Chartiers and Union townships, and the larger part of Baldwin, in addition to its present area. The population in 1860 was 4,617; in 1870, 5,322; in 1880, 2,329.

The thickly settled district is known as Mount Oliver, in the northern and western part of the township. It is conveniently accessible by street railways and inclined planes. There are no manufactures of any importance, and the people find employment in the city. The extensive coal-works at Beck’s run are owned by the estate of James H. Hays. Frederick Hampe, the first postmaster at Mount Oliver, was appointed in February, 1874. He was succeeded by John Conrad in September, 1885.

St. Joseph’s German Catholic Church, Mount Oliver, was dedicated November 20, 1870. The cornerstone was laid October 4, 1868, and the first movement toward the building project was made July 12th preceding. The site was secured prior to the civil war, and fortified for the protection of the city. The membership was formerly connected with St. Michael’s, and was separately organized by Father Luke. The cemetery of St. Peter’s Church is also in this township. St. Paul’s congregation is the only other religious body, but neighboring churches of other denominations are largely represented by their membership.