There are 9 US Post Offices in Mercer County serving a total of 40,603 residents equating to 4,511 residents per post office. It's estimated that approximately 64,807 packages are handled across the 9 county post offices annually. Mercer County is a county located in the U.S. state of Ohio. As of the 2010 census, the population was 40,814. Its county seat is Celina. The county was created in 1820 and later organized in 1824. It is named for Hugh Mercer, an officer in the American Revolutionary War. Mercer County comprises the Celina, OH Micropolitan Statistical Area, which is also included in the Lima-Van Wert-Celina, OH Combined Statistical Area."