Bedford Township was formed in 1767 within Cumberland County. Then, in 1775, Frankstown Township was formed out of the northern third of Bedford Township, along with a portion of Barre Township. In 1785, Woodberry Township was formed out of the southern part of Frankstown Township along with an additional small portion of Bedford Township.
In 1794, St. Clair Township was formed from a portion of Bedford Township; it was located adjacent to the western half of Woodberry and bordered along that township's southern boundary. In 1798, that western part of Woodberry Township (lying to the west of Dunnings Mountain range, between it and the Allegheny Mountain range) was separated to be formed into Greenfield Township.
Thirty-six years after the formation of Greenfield Township, and forty after St. Clair Township was formed, Greenfield was divided in two along an east-west line and St. Clair was divided similarly in two along an east-west line although, in both cases, the two halves were not the same size. The southern third of Greenfield and the northern quarter of St. Clair were combined into a new township that was named: Union. In 1846, when Blair County was erected out of Huntingdon and Bedford, the remaining two-thirds of Greenfield were absorbed into the new county. The entire region that had been formed as Union remained under the jurisdiction of Bedford County.
In 1812, St. Clair Township was again divided into two parts, the southern two-thirds being formed into a new township: Napier. The northern third of St. Clair remained as a single township until 1875. In that year, the remaining St. Clair Township was divided into two approximately equal sized parts by a north-south line that followed along the summit of Chestnut Ridge. Both new townships retained the name of St. Clair, simply adding the word East to the one and West to the other.
Before its division, St. Clair Township was named in honor of Arthur St. Clair.
