
Jefferson County
- State:
- Illinois
- County Seat:
- Mount Vernon
Jefferson County's estimated 2025 population is 36,122 with a growth rate of -0.27% in the past year according to the most recent United States census data. Jefferson County is the 38th largest county in Illinois. The 2010 population was 38,757 and has seen a growth of -6.8% since that time.