There are 9 US Post Offices in Union County serving a total of 57,292 residents equating to 6,366 residents per post office. It's estimated that approximately 91,444 packages are handled across the 9 county post offices annually. Union County is a county located in the US state of Ohio. As of the 2010 census, the population was 52,300. Its county seat is Marysville. Its name is reflective of its origins, it being the union of pieces of Franklin, Delaware, Madison, and Logan counties. Union County is part of the Columbus, OH Metropolitan Statistical Area."