Marion County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 65,359. Its county seat is Marion. The county was erected by the state of Ohio on February 20, 1820 and later reorganized in 1824. It is named for General Francis "The Swamp Fox" Marion, a South Carolinian officer in the Revolutionary War. Marion County comprises the Marion, OH Micropolitan Statistical Area, which is also included in the Columbus-Marion-Zanesville, OH Combined Statistical Area.