11 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Centralia | 1 | 11,645 | 12,152 | -0.86% | 1,420 | 8.20 | City |
| Salem | 2 | 6,917 | 7,264 | -0.99% | 989 | 6.99 | City |
| Sandoval | 3 | 1,108 | 1,155 | -0.81% | 1,112 | 1 | Village |
| Odin | 4 | 892 | 936 | -1% | 889 | 1 | Village |
| Kinmundy | 5 | 705 | 732 | -0.7% | 588 | 1.20 | City |
| Patoka | 6 | 498 | 523 | -0.99% | 447 | 1.11 | Village |
| Iuka | 7 | 484 | 508 | -1.02% | 660 | 0.73 | Village |
| Alma | 8 | 299 | 314 | -0.99% | 264 | 1.13 | Village |
| Kell | 9 | 165 | 174 | -1.2% | 165 | 1 | Village |
| Vernon | 10 | 97 | 101 | -1.02% | 110 | 0.88 | Village |
| Walnut Hill | 11 | 91 | 96 | -1.09% | 239 | 0.38 | Village |
Centralia is the largest city in Marion County, with a population of 11,645.
Walnut Hill is the smallest city in Marion County.
There are 11 cities in Marion County, Illinois.