Warren County is a county located in the southwestern part of the U.S. state of Ohio. As of the 2020 census, the population was 242,337. Its county seat is Lebanon and most populous city is Mason. The county is one of Ohio's most affluent, with the second highest median income of the state's 88 counties. The county was established on May 1, 1803, from Hamilton County; it is named for Dr. Joseph Warren, a hero of the Revolution who sent Paul Revere and the overlooked William Dawes on their famous rides and who died at the Battle of Bunker Hill. Warren County is part of the Cincinnati, OH-KY-IN Metropolitan Statistical Area.