Mifflin Township was named in honor of Thomas Mifflin, a colonial leader during the years of the American Revolution who became the first governor of Pennsylvania. His name was given to many places in the state, including Mifflin Townships in Lycoming County, Dauphin County, and Columbia County. The land that comprised Mifflin Township is in the northwestern section of Cumberland County. The township came into being in 1797 as the result of growing settlement within the county. Mifflin Township was divided away from Hopewell Township to assist in the smooth function of county administration. The township was bounded by the North Mountains to the north, the Conodoguinet Creek to the south, Frankford Township to the east, and Hopewell Township to the west. This political division existed for almost a century, when, again because of population growth, the township was divided into Upper Mifflin Township and Lower Mifflin Township in 1892.
The original township in Cumberland County was largely wooded with the soil a mixture of clay, gravel, and slate. There are four significant streams flowing through the township area from the mountains to the Conodoguinet. These are Three-Square Hollow Run, Brandy Run, Whiskey Run, and Big Run (now known as Doubling Gap Creek). Long before European settlers arrived, the native Indians established well-used major trails through the township. It is believed that there was an old Indian burial ground between the branches of Brandy Creek (Run) as well as a village. Tradition also says that the native population grew corn on the long finger of land formed by a deep bend of the Conodoguinet.1
Most of the newly arrived settlers in the area had origins in Scotland and Ireland and came to the township in the 1720s and 1730s. William Thompson settled on the great bend of the Creek mentioned above, and Andrew McElwain (or McIlvaine) settled on the "Fountain of Health" farm. They were joined by other Presbyterians with family names such as Stevenson, Shannon, Carahan, Nicholson, Williamson, and others.2 Living in the township was not without danger. Firearms were carried to worship services and kept close by when working away from the cabins. Indian attacks did take place, especially during the French and Indian War. Attacks involving the Williamson and the Nicholson families are two noteworthy incidents. Early Mifflin Township apparently had several small forts or block houses where residents gathered when attacks were threatened, but in the incidents mentioned, there was no warning.3
Settlers harvested the trees, farmed the land, and erected grist and saw mills, as well as many whiskey stills. Agriculture was the basis of life through the time when the township split occurred. In the latter decades of the nineteenth century, the growing population stressed the township's administrative efficiency, and the large township was divided into two smaller areas in 1892.