St. Clair County once stretched from Fort Massac on the Ohio River in a line northwest to the present site of Peoria, followed the Illinois River to the banks of the Mighty Mississippi and continued on to the Ohio River at Fort Massac. The area was part of the Northwest Territory which was established by Congress in 1787.
Arthur St. Clair, became the Governor of the Northwest Territory. Two years later, St. Clair paid a visit to what is now called southern Illinois. On April 27, 1790 Governor St. Clair laid out the boundaries for the county which now bears his name. He established the county seat in Cahokia. The photo to the right is the Church of The Holy Family in Cahokia. It is the oldest church west of the Allegheny Mountains. The church was established in 1699 by Catholic missionaries from the seminary of Quebec.
By 1820, St. Clair County had more than 5,000 residents; its population did not reach 10,000 until around 1835. After the Civil War, there were more than 50,000 citizens in St. Clair County. Today, St. Clair County is home to 270,056 people, according to the U.S. Census Bureau.
St. Clair County is home to Scott Air Force. Scott Air Force Base pumps about $2 billion a year into the region's economy. With more than 13,000 employees, Scott is one of the top five employers in the St. Louis region.
St. Clair County is surrounded by several major tourist attractions including the National Shrine of Our Lady of the Snows, Gateway International Raceway and Cahokia Mounds.