There are 7 US Post Offices in Union County serving a total of 17,337 residents equating to 2,477 residents per post office. It's estimated that approximately 27,672 packages are handled across the 7 county post offices annually. Union County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 17,808. Its county seat is Jonesboro. It is located in the southern portion of Illinois known locally as \"Little Egypt\"."