Wilmington is a city in and the county seat of Clinton County, Ohio, United States. The population was 12,664 at the 2020 census. It is the princi…Wilmington is a city in and the county seat of Clinton County, Ohio, United States. The population was 12,664 at the 2020 census. It is the principal city of the Wilmington micropolitan area, which includes all of Clinton County and is part of the greater Cincinnati–Wilmington–Maysville combined statistical area.