City | Rank | 2026 Pop.↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Centralia | 1 | 11,805 | 12,118 | -0.49% | 1,440 | 8 | City |
| Salem | 2 | 6,972 | 7,275 | -0.9% | 997 | 7 | City |
| Sandoval | 3 | 1,114 | 1,161 | -0.8% | 1,118 | 1 | Village |
| Central City | 4 | 1,059 | 1,103 | -0.75% | 1,874 | 1 | Village |
| Odin | 5 | 896 | 936 | -0.78% | 893 | 1 | Village |
| Kinmundy | 6 | 696 | 727 | -0.85% | 581 | 1 | City |
| Patoka | 7 | 502 | 529 | -0.99% | 450 | 1 | Village |
| Iuka | 8 | 487 | 507 | -0.81% | 664 | 1 | Village |
| Alma | 9 | 299 | 311 | -0.99% | 264 | 1 | Village |
| Kell | 10 | 167 | 174 | -1.18% | 167 | 1 | Village |
| Vernon | 11 | 96 | 101 | -1.03% | 109 | 1 | Village |
| Walnut Hill | 12 | 91 | 96 | -1.09% | 239 | 0 | Village |
Centralia is the largest city in Marion County, with a population of 11,805.
Walnut Hill is the smallest city in Marion County, with a population of 91.
There are 12 cities in Marion County, Illinois.