 Before European settlement the area was inhabited by the Akhrakouaeronon or Atrakouaehronon, a subtribe of the Susquehannock.By 1813 the area once comprising the sprawling county of Northumberland had been divided over time and allotted to other counties such that lands once occupied by Old Northumberland at its greatest extent are now found in Centre, Columbia, Luzerne, Lycoming, Mifflin, Union, Clearfield, Clinton, Montour, Bradford, Lackawanna, Susquehanna, Wyoming, Tioga, Potter, McKean, Warren, Venango, Snyder, and Schuylkill Counties.According to the U.S. Census Bureau, the county has a total area of , of which is land and (4.0%) is water.