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.