There are 5 US Post Offices in Union County serving a total of 15,547 residents equating to 3,109 residents per post office. It's estimated that approximately 24,815 packages are handled across the 5 county post offices annually. Union County is a county located in the U.S. state of Kentucky. As of the 2010 census, the population was 15,007. Its county seat is Morganfield. The county was formed on January 15, 1811. Located at the mouth of the Wabash River, Union County, along with neighboring Posey County, Indiana, and Gallatin County, Illinois form the tri-point of the Illinois-Indiana-Kentucky Tri-State Area.