Alton is a city on the Mississippi River in Madison County, Illinois, United States, about 18 miles north of St. Louis, Missouri. The population w…Alton is a city on the Mississippi River in Madison County, Illinois, United States, about 18 miles north of St. Louis, Missouri. The population was 25,676 at the 2020 census. It is a part of the River Bend area in the Metro-East region of the Greater St. Louis metropolitan area.