St. Clair County is the oldest county in the U.S. state of Illinois; its western border is formed by the Mississippi River, bordering the U.S. state of Missouri.
It is a part of the Metro East in southern Illinois.
According to the 2010 United States 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 by the government of the Northwest Territory, before the establishment of Illinois as a state.
Cahokia Village in the county was founded in 1697 and was a French settlement and former Jesuit mission.

St. Clair County is part of the American Bottom or Metro-East area of the St. Louis, MO-IL Metropolitan Statistical Area.

In 1970, the United States Census Bureau placed the mean center of U.S. population in St. Clair County.
St. Clair County is the oldest county in the U.S. state of Illinois; its western border is formed by the Mississippi River, bordering the U.S. state of Missouri. It is a part of the Metro East in southern Illinois. According to the 2010 United States 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 by the government of the Northwest Territory, before the establishment of Illinois as a state. Cahokia Village in the county was founded in 1697 and was a French settlement and former Jesuit mission.
St. Clair County is part of the American Bottom or Metro-East area of the St. Louis, MO-IL Metropolitan Statistical Area.
In 1970, the United States Census Bureau placed the mean center of U.S. population in St. Clair County.
St. Clair County, Illinois