12 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marion | 1 | 17,116 | 16,831 | 0.28% | 1,188 | 14 | City |
| Herrin | 2 | 12,207 | 12,345 | -0.19% | 1,262 | 10 | City |
| Carterville | 3 | 5,803 | 5,856 | -0.15% | 1,095 | 5 | City |
| Johnston City | 4 | 3,274 | 3,379 | -0.52% | 1,547 | 2 | City |
| Cambria | 5 | 1,914 | 1,551 | 3.29% | 1,353 | 1 | Village |
| Energy | 6 | 950 | 968 | -0.31% | 795 | 1 | Village |
| Hurst | 7 | 746 | 758 | -0.27% | 841 | 1 | City |
| Pittsburg | 8 | 557 | 569 | -0.36% | 270 | 2 | Village |
| Creal Springs | 9 | 494 | 505 | -0.4% | 481 | 1 | City |
| Freeman Spur | 10 | 269 | 275 | -0.37% | 674 | 0 | Village |
| Spillertown | 11 | 175 | 180 | -0.57% | 330 | 1 | Village |
| Colp | 12 | 173 | 178 | -0.57% | 1,291 | 0 | Village |
Marion is the largest city in Williamson County, with a population of 17,116.
Colp is the smallest city in Williamson County, with a population of 173.
There are 12 cities in Williamson County, Illinois.