There are 20 US Post Offices in Northumberland County serving a total of 82,589 residents equating to 4,129 residents per post office. It's estimated that approximately 131,821 packages are handled across the 20 county post offices annually. Northumberland County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 94,528. Its county seat is Sunbury. The county was formed in 1772 from parts of Lancaster, Berks, Bedford, Cumberland, and Northampton Counties and named for the county of Northumberland in northern England. Northumberland County is a fifth class county according to the Pennsylvania's County Code. Northumberland County comprises the Sunbury, PA Micropolitan Statistical Area, which is also included in the Bloomsburg-Berwick-Sunbury, PA Combined Statistical Area. Among its famous residents, Joseph Priestley, the enlightenment chemist and theologian, left England in 1796 due to religious persecution and settled on the Susquehanna River. His former house (originally purchased by chemists from Pennsylvania State University after a colloquium that founded the American Chemical Society) is a historical museum."