Mercer County is located in the U.S. state of Ohio. As of the 2020 census, the population was 42,528. 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.

Celina is a city in and the county seat of Mercer County, Ohio, United States about 58 miles (93 km) northwest of Dayton. The population was 10,935 at the 2020 census. Celina is situated on the northwestern shores of Grand Lake St. Marys.