|St. Clair County Illinois Cities/Towns -
Alorton, Belleville (County Seat), Brooklyn, Cahokia, Caseyville, Centreville, Darmstadt, Dupo, East Carondelet, East St. Louis, Fairview Heights, Fayetteville, Floraville, Freeburg, Lebanon, Lenzburg, Marissa, Mascoutah, Millstadt, New Athens, O'Fallon, Paderborn, Sauget, Shiloh, Smithton, St. Libory, Summerfield, Swansea, Washington Park
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St. Clair County is a county located in the U.S. state of Illinois. As of 2016, the population was 262,759. Its county seat is Belleville.
St. Clair County is also the first county established in the state of Illinois. Its existence actually precedes the state of Illinois as it was established in the Northwest Territory in 1790. The county was named in honor of Arthur St. Clair, Governor of the Northwest Territory, who organized the county in 1790. In its original boundaries, the county covered two-thirds of present-day Illinois.