St. Clair County Illinois is located east of Downtown St. Louis with the Mississippi River being the western boundary of the county. Madison County is to the north and Monroe County is to the south. St. Clair County was founded in 1790 when it was then part of the Northwest Territory. Later in 1809, it was the one of the two original counties to be formed in the Illinois Territory. At that time, it covered the northern three-fourths of the what is the state of Illinois today. The area that St. Clair covers today, is 674 square miles. The county seat is located in Belleville Illinois and the 2010 US Census put the population at 270,056.
Below is a list of the larger communities that are found in St. Clair County. Click on any of the links to find that community's specific government information such as the address to the city hall, phone numbers of the mayor, city clerk, police department and other information.