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. Collinsville is the location of Cahokia Mounds State Historic Site, a National Historic Landmark and UNESCO World Heritage Site.