Resident Population in St. Clair County, MO was 9.75200 Thous. of Persons in January of 2023, according to the United States Federal Reserve. Historically, Resident Population in St. Clair County, MO reached a record high of 9.82300 in January of 2010 and a record low of 7.66700 in January of 1970. Trading Economics provides the current actual value, an historical data chart and related indicators for Resident Population in St. Clair County, MO - last updated from the United States Federal Reserve on May of 2024.