Created on March 12, 1800, from parts of Allegheny, Westmoreland, and Lycoming Counties and was named for General John Armstrong. It was attached to Westmoreland County until 1805. Kittanning, the county seat was incorporated as a borough on April 2, 1821, and derived its name from a Delaware Indian village at the same place.

     The area saw much fighting between settlers and Indians in the French and Indian War and Revolutionary War. John Armstrong’s attack on the Indian village of Kittanning in 1756 proved that the Indians were not invincible. The territory was purchased from the Indians by the two Treaties of Fort Stanwix, 1768 (known as the New Purchase) and 1784 (known as the Last Purchase). The State’s third ranked bituminous coal producing county, Armstrong also has produced glass, clay, brick, and quarried stone. Most of the coal mines are subsurface. A brief oil boom occurred after 1865, and natural gas was produced long afterwards. Steel production was once important, but declined by the 1930s. Armstrong’s farming is strong in livestock and has a valuable mushroom crop. Thirty percent of the land is farmed.