Union County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 42,681. Its county seat is Lewisburg. The county was created on March 22, 1813, from part of Northumberland County. Its name is an allusion to the federal Union. Mifflinburg was established by legislation as the first county seat until it was moved to New Berlin in 1815. Lewisburg became county seat in 1855 and has remained so since.