St. Clair was one of the original townships of Allegheny County, Pennsylvania at the county's creation in 1788. In 1836, the St. Clair Township was divided into two separate townships, Upper St. Clair and Lower St. Clair. The residents of Upper St. Clair formed their township to ensure better government service that could be obtained by separating from the more densely populated northern part of the township. Upper St. Clair Township was further subdivided throughout the 19th and 20th centuries as several parts of the original township separated to form new townships and boroughs. In 1973, Upper St. Clair Township adopted a home rule charter that took effect on January 5, 1976, and is no longer governed by the Pennsylvania Township Code.