The township was named after Arthur St. Clair, an American Revolutionary War general and president of the Continental Congress in 1787.

It was bounded on the west by Chartiers Creek and on the east partially by Streets Run.

The original Lower St. Clair Township consisted of the present-day neighborhoods of the City of Pittsburgh south of the Monongahela and Ohio, and all or parts of Green Tree, Carnegie, Baldwin, Bridgeville, Dormont, Mount Lebanon and other boroughs and townships in the South Hills.

The township was subdivided several times, and today exists only as the neighborhood of St. Clair in the City of Pittsburgh.

Upper St. Clair Township, subdivided many times, still exists in the South Hills of Pittsburgh.