St. Clair County became Illinois’ first county 225 years ago this week. At the time, the county included most of present-day Illinois, running from today’s northern border with Wisconsin to Southern Illinois.

The county’s founding occurred before Illinois was officially admitted to the Union in 1818. St. Clair County was thus incorporated as a part of the Northwest Territory, which encompassed portions of modern-day Illinois, Indiana, Wisconsin, Minnesota, Michigan and Ohio.

From its establishment in 1809 until 1827, St. Clair County drastically shrank in size to its current 674 square miles of land in Southwestern Illinois.

In honor of the founding anniversary, St. Clair County leaders held a full board meeting yesterday at the Old Cahokia Courthouse, a state historic site which served as the county’s judicial center in the 18th Century. 

Learn more:
Origin and Evolution of Illinois Counties (Illinois Secretary of State)
St. Clair County Historical Society
Illinois Historic Preservation Agency