St. Clair County is the oldest
county in the
U.S. state of
Illinois. According to the
2010 census, it had a population of 270,056, making it the eighth most populous county in Illinois and the most populous in the
southern portion of the state. Its
county seat is
Belleville. The county was founded in 1790, before the establishment of Illinois itself, and contains the historic French settlement and former mission,
Cahokia Village, founded 1697.