Thomas Hartley, Michael Graybill, George Leffler, and John Kern all received warrants for land on February 11, 1773.

Marcus Huling appears to have been the first permanent settler in Mosquito Valley, which makes up the greatest portion of Armstrong Township.

Upon Thomas Huling's death, the land was foreclosed upon and left to be reclaimed by nature until the arrival of German farmers in 1832.

Farmers moved into the area and began several successful orchard and dairy operations after the valley was cleared of trees.

It acquired water rights to the streams and bought up most of the farmland in the southwestern portion of Armstrong Township in the early 1920s as part of its watershed.

Visitors to the "water company" lands can see the stone remains of the early settlers homesteads spread throughout the watershed.

This mass purchase of farmland in Armstrong County and the ceding of territory to neighboring boroughs has accounted for the steep decline in population since 1890.

With the construction of this plant the lands of the water authority were opened as a nature preserve to the general public.

Bald Eagle Mountain is in the northern part of the township, and the crest of North White Deer Ridge forms the southern border.

Like Mosquito Creek, Hagermans Run cuts north through Bald Eagle Mountain, but passes through South Williamsport before reaching the West Branch.

Pennsylvania Route 554 runs through the Hagermans Run valley, leading north to South Williamsport and Williamsport, and south over North White Deer Ridge 8 miles (13 km) to Elimsport in Washington Township.

According to the United States Census Bureau, Armstrong Township has a total area of 25.5 square miles (66.1 km2).