Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Union County is a county located in the U.S. state of Pennsylvania. As of 2000, the population was 41,624. Union County was created on March 22, 1813, from part of Northumberland County. Its name is an allusion to the federal Union. Its county seat is Lewisburg.According to the U.S. Census Bureau, the county has a total area of 317 square miles (821 km²), of which, 317 square miles (820 km²) of it is land and 0 square miles (1 km²) of it (0.12%) is water.