St. Clair County Administrative Offices are located at 10 Public Square, Belleville IL 62220; phone: 217-774-4421.
Formed in 1790, the original county seat was at Cahokia; moved to Belleville in 1814; named for Governor of the "Northwest Territory" Arthur St. Clair (1736-1818).
April 27, 1790, Governor St. Clair issued his proclamation organizing St. Clair as a county of the Northwest Territory. It had for its boundaries a direct line from the mouth of the Little Mackinaw to the mouth of the Massac creek, thence down the Ohio to the Mississippi, up the Mississippi to the mouth of the Illinois and up the Illinois to the mouth of the Mackinaw. As thus constituted the county extended nearly two hundred and fifty miles from north to south with a maximum width of about eighty miles. It embraced the territory of twenty existing counties and fractions of eleven others. But with all this wealth of territory, St. Clair was a small county as compared with Knox, created by proclamation June 20 of the same year, which included about half of the State of Illinois, the whole of Indiana, that part of Ohio west of the Great Miami River, the greater part of Michigan, and a considerable part of Wisconsin as these states exist at present. Knox was organized to meet the wants of the settlements about Vincennes.