South Franklin Township has been a municipality since 1892 with several changes. It was first organized as Amwell Township(1781/82) when Washington County was founded, but in 1788 Morris Township formed to the south. Amwell was further divided four years later when Canton Township was founded in the north. In 1855, Franklin Township was formed from parts of Amwell, Canton and Morris Townships, and on Feb. 8, 1892, Franklin Township divided into North and South Franklin Townships.

According to the history of Washington County, South Franklin had six schools at the turn of the century which is now McGuffey School District. The main highway through the Township was built in 1851 on parts of what is now Route 18 and Old Plank Road. It was built by Upper Ten-Mile Plank Road Co. Prosperity Plank Road ran from Washington to Prosperity. It was a toll road until 1906, when it was condemned and the toll discontinued.

The township lost some ground when Green Hills Borough seceded in 1978, taking with it about 600 acres, mostly by Lone Pine Golf Club. The borough was formed after the Township voters defeated a referendum the previous year that would have allowed alcohol to be sold in the dry township. Even though it is no longer in South Franklin Township, the beauty of the golf course and current facilities add to the richness of the area.

Population has grown as well as an improved road system, with a census in 1970 of 1,730 residents to the 2000 census of 3,796. Most of the growth occurred in 1980 from the 1,730 to 3,540. Much of the agricultural land has been subdivided by its owners, with many of their children building homes on their parents property.

The township has installed several miles of city water and taking a hard look at sewage for at least some of the township. With those infrastructure additions, the township may grow even more. As of 2007, major developments are planned on the north around dams three and four (part of reservoir four is in South Franklin Township), and in Green Hills Boorough.

In 1946, air travel came to the Township when the first runway was built at Washington County Airport in the northern part of South Franklin Township. Today, the airport consists of 130 employees, 365 acres of land, 78 aircraft, 6+ corporate hangers, 37 T-hangers, and 40,000 annual take offs and landings. The Airport also has available services such as major aircraft airframe and engine repair, flight instruction, air charter services, aircraft sales and rentals, and aircraft storage facilities. Future expansion plans are currently being developed.