11 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Centralia | 1 | 11,608 | 12,144 | -0.76% | 1,416 | 8 | City |
| Salem | 2 | 6,887 | 7,274 | -0.92% | 985 | 7 | City |
| Sandoval | 3 | 1,109 | 1,157 | -0.72% | 1,113 | 1 | Village |
| Odin | 4 | 886 | 936 | -0.89% | 883 | 1 | Village |
| Kinmundy | 5 | 701 | 731 | -0.71% | 585 | 1 | City |
| Patoka | 6 | 499 | 526 | -0.8% | 448 | 1 | Village |
| Iuka | 7 | 481 | 507 | -0.82% | 656 | 1 | Village |
| Alma | 8 | 301 | 313 | -0.66% | 266 | 1 | Village |
| Kell | 9 | 168 | 174 | -0.59% | 168 | 1 | Village |
| Vernon | 10 | 99 | 102 | 0% | 112 | 1 | Village |
| Walnut Hill | 11 | 91 | 96 | -1.09% | 239 | 0 | Village |
Centralia is the largest city in Marion County, with a population of 11,608.
Walnut Hill is the smallest city in Marion County, with a population of 91.
There are 11 cities in Marion County, Illinois.