The county was organized in 1841 and named after General Arthur St. Clair, Governor of the Northwest Territory.

[3] St. Clair was also the 9th president of the United States in Congress Assembled.

Under his presidency, the Northwest Ordinance and United States Constitution were passed.

[4] According to the April 1907 issue of The Century Magazine, for well over 30 years (dating from 1870) St. Clair County was in open rebellion against the U.S. Government, refusing to pay interest on bonds which it had issued to assist in the building of a railroad, which was never built.

[5] As of the census[11] of 2000, there were 9,652 people, 4,040 households, and 2,791 families residing in the county.

27.40% of all households were made up of individuals, and 14.80% had someone living alone who was 65 years of age or older.