St. Clair County, Missouri was organized January 29, 1841 and named for Revolutionary War General Arthur St. Clair. The present part of the county north of the Osage River was once part of Rives (now Henry County).
Osceola is the county seat. The county form of government is county organization, and St. Clair County is in the Southern District of the Missouri Court of Appeals (Springfield). The present county population is 9,652.
The principal towns, their population, zip codes and prefixes are:
| Town | Population | Zip Code | Telephone Prefix |
| Osceola | 835 | 64776 | (417) 646 |
| Appleton City | 1314 | 64724 | (660) 476 |
| Lowry City | 728 | 64763 | (417) 644 |
| Collins | 176 | 64738 | (417) 275 |
St. Clair County is in the Southern District of the Missouri Court of Appeals.
[27th Circuit][Bates County][Henry County][St. Clair County]