Map - St. Clair County, Illinois (Saint Clair County)

St. Clair County is the oldest county in Illinois; its western border is formed by the Mississippi River, bordering Missouri. It is a part of the Metro East in southern Illinois. At the 2020 United States census, it had a population of 257,400, making it the ninth-most populous county in Illinois. Its county seat is Belleville. The county was founded in 1790 by the government of the Northwest Territory, from the western half of Knox County, before the establishment of Illinois as a state as the base for the Illinois Territory. 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.

This area was occupied for thousands of years by cultures of indigenous peoples. The first modern explorers and colonists of the area were French and French Canadians, founding a mission settlement in 1697 now known as Cahokia Village. After Great Britain defeated France in the Seven Years' War in 1763 and absorbed its territory in North America east of the Mississippi River, British-American colonists began to move into the area. Many French Catholics moved to settlements west of the river rather than live under British Protestant rule.

After the United States achieved independence in the late 18th century, St. Clair County was the first county established in present-day Illinois; it antedates Illinois' existence as a separate jurisdiction. The county was established in 1790 by a proclamation of Arthur St. Clair, first governor of the Northwest Territory, who named it after himself.

The original boundary of St. Clair county covered a large area between the Mackinaw and Ohio rivers. In 1801, Governor William Henry Harrison re-established St. Clair County as part of the Indiana Territory, extending its northern border to Lake Superior and the international border with Rupert's Land.

When the Illinois Territory was created in 1809, Territorial Secretary Nathaniel Pope, in his capacity as acting governor, issued a proclamation establishing St. Clair and Randolph County as the two original counties of Illinois.

Originally developed for agriculture, this area became industrialized and urbanized in the area of East St. Louis, Illinois, a city that developed on the east side of the Mississippi River from St. Louis, Missouri. It was always strongly influenced by actions of businessmen from St. Louis, who were initially French Creole fur traders with western trading networks.

In the 19th century, industrialists from St. Louis put coal plants and other heavy industry on the east side of the river, developing East St. Louis. Coal from southern mines was transported on the river to East St. Louis, then fed by barge to St. Louis furnaces as needed. After bridges spanned the river, industry expanded.

In the late 19th and early 20th centuries, the cities attracted immigrants from southern and eastern Europe and from the South. In 1910 there were 6,000 African Americans in the city. With the Great Migration underway from the rural South, to leave behind Jim Crow and disenfranchisement, by 1917, the African-American population in East St. Louis had doubled. Whites were generally hired first and given higher–paying jobs, but there were still opportunities for American blacks. If hired as strikebreakers, they were resented by white workers, and both groups competed for jobs and limited housing in East St. Louis. The city had not been able to keep up with the rapid growth of population. The United States was developing war industries to support its eventual entry into the Great War, now known as World War I.