Mason is a city in southwestern Warren County, Ohio, United States, approximately 22 miles (35 km) from downtown Cincinnati. As of the 2010 census, Mason's population was 30,712.Mason was transformed into a large, bustling community, and one of the most affluent in Greater Cincinnati, beginning in the 1990s. Mason sits at the core of the Cincinnati-Dayton Metropolitan Region, the 14th largest urban area in the nation.