COUNTY · IL
Williamson County, Illinois
Williamson County is a county in Illinois with FIPS code 17199 and an estimated population of 66,706 (Census 2023), 22nd largest of 102 in the state.
County facts
- Name
- Williamson County
- State
- Illinois
- Type
- county
- FIPS code
- 17199
- State FIPS
- 17
- Population (2023)
- 66,706
- Change since 2020
- -0.6%
- State rank
- 22 of 102
Population is the Census Bureau’s Vintage 2023 estimate.
Questions
What is the FIPS code for Williamson County?
Williamson County, Illinois has the five-digit FIPS code 17199: 17 for the state and 199 for the county.
What state is Williamson County in?
Williamson County is in Illinois.
How do I know if an address is in this county?
Use the address lookup. ZIP codes and city names can cross county lines, so an address is the reliable check.
Sources: U.S. Census Bureau geographic reference files; Census Bureau Population Estimates, Vintage 2023. Last reviewed 2026-10-27.