St. Clair County is a county located in the western portion of the U.S. state of Missouri. As of the 2020 census, the population was 9,284. Its county seat is Osceola. The largest city is Appleton City. The county was organized in 1841 and named after General Arthur St. Clair, Governor of the Northwest Territory. 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.