Mason City is a city in and the county seat of Cerro Gordo County, Iowa, United States. The population was 28,079 in the 2010 census, a decline from 29,172 in the 2000 census. The Mason City Micropolitan Statistical Area includes all of Cerro Gordo and Worth counties. It is commonly referred to as the "River City", as the city grew up centered on the Winnebago River.