Mason City is a city and the county seat of Cerro Gordo County, Iowa, United States. The population was 27,338 in the 2020 census. Maso…Mason City is a city and the county seat of Cerro Gordo County, Iowa, United States. The population was 27,338 in the 2020 census. Mason City is known for its musical heritage, high concentrations of renowned Prairie School style architecture, and a close connection with nearby Clear Lake.