16 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Marion | 1 | 16,860 | 16,840 | 0.02% | 1,171 | 14 | City |
| Herrin | 2 | 12,070 | 12,339 | -0.45% | 1,248 | 10 | City |
| Carterville | 3 | 5,778 | 5,852 | -0.26% | 1,090 | 5 | City |
| Marion | 4 | 3,866 | 3,902 | -0.1% | 86 | 45 | Township |
| Johnston City | 5 | 3,260 | 3,379 | -0.73% | 1,541 | 2 | City |
| Cambria | 6 | 1,913 | 1,552 | 3.91% | 1,352 | 1 | Village |
| Energy | 7 | 941 | 970 | -0.63% | 787 | 1 | Village |
| Hurst | 8 | 743 | 758 | -0.4% | 838 | 1 | City |
| Pittsburg | 9 | 554 | 566 | -0.36% | 268 | 2 | Village |
| Creal Springs | 10 | 491 | 505 | -0.61% | 478 | 1 | City |
| Freeman Spur | 11 | 265 | 274 | -0.75% | 664 | 0 | Village |
| Crab Orchard | 12 | 241 | 222 | -2.43% | 173 | 1 | CDP |
| Whiteash | 13 | 236 | 83 | 7.27% | 271 | 1 | CDP |
| Marion | 14 | 210 | 217 | -0.94% | 6 | 36 | Township |
| Spillertown | 15 | 175 | 180 | -0.57% | 545 | 0 | Village |
| Colp | 16 | 173 | 178 | -0.57% | 1,291 | 0 | Village |
Marion is the largest city in Williamson County, with a population of 16,860.
Colp is the smallest city in Williamson County, with a population of 173.
There are 16 cities in Williamson County, Illinois.