Thornton is a city in Cerro Gordo County, Iowa, United States. The population was 400 at the time of the 2020 census. It is part of the M…Thornton is a city in Cerro Gordo County, Iowa, United States. The population was 400 at the time of the 2020 census. It is part of the Mason City Micropolitan Statistical Area. Thornton, Iowa is located at 42°56′42″N 93°23′3″W / 42.94500°N 93.38417°W.