Collinsville is a city located mainly in Madison County and partially in St. Clair County, Illinois, United States. As of the 2020 census, the city h…Collinsville is a city located mainly in Madison County and partially in St. Clair County, Illinois, United States. As of the 2020 census, the city had a population of 24,366. Collinsville is approximately 14 miles east of St. Louis, Missouri, and is part of that city's Metro East area.