31 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Vernon | 1 | 72,202 | 71,295 | 0.54% | 2,015 | 36 | Township |
| Centralia | 2 | 13,758 | 14,311 | -0.59% | 392 | 35 | Township |
| Centralia | 3 | 11,697 | 12,144 | -0.76% | 1,426 | 8 | City |
| Salem | 4 | 8,721 | 9,085 | -0.59% | 246 | 35 | Township |
| Salem | 5 | 6,951 | 7,274 | -0.93% | 994 | 7 | City |
| Sandoval | 6 | 2,139 | 2,203 | -0.33% | 122 | 18 | Township |
| Odin | 7 | 1,516 | 1,579 | -0.66% | 84 | 18 | Township |
| Raccoon | 8 | 1,492 | 1,538 | -0.4% | 42 | 35 | Township |
| Stevenson | 9 | 1,135 | 1,180 | -0.53% | 31 | 37 | Township |
| Sandoval | 10 | 1,117 | 1,157 | -0.71% | 1,121 | 1 | Village |
| Kinmundy | 11 | 1,082 | 1,120 | -0.46% | 30 | 37 | Township |
| Iuka | 12 | 982 | 1,019 | -0.51% | 27 | 36 | Township |
| Tonti | 13 | 980 | 1,017 | -0.51% | 28 | 35 | Township |
| Salem | 14 | 923 | 943 | 0.11% | 26 | 36 | Township |
| Patoka | 15 | 903 | 938 | -0.55% | 26 | 35 | Township |
| Odin | 16 | 894 | 936 | -1% | 891 | 1 | Village |
| Haines | 17 | 839 | 867 | -0.36% | 23 | 36 | Township |
| Alma | 18 | 756 | 784 | -0.53% | 21 | 37 | Township |
| Kinmundy | 19 | 706 | 731 | -0.7% | 589 | 1 | City |
| Patoka | 20 | 503 | 526 | -0.98% | 451 | 1 | Village |
| Iuka | 21 | 485 | 507 | -1.02% | 662 | 1 | Village |
| Romine | 22 | 477 | 493 | -0.42% | 13 | 36 | Township |
| Omega | 23 | 426 | 447 | -0.93% | 12 | 35 | Township |
| Carrigan | 24 | 350 | 364 | -0.57% | 10 | 35 | Township |
| Foster | 25 | 344 | 362 | -1.15% | 10 | 36 | Township |
| Salem | 26 | 338 | 340 | 0.3% | 10 | 35 | Township |
| Meacham | 27 | 334 | 350 | -0.6% | 9 | 37 | Township |
| Alma | 28 | 303 | 313 | -0.66% | 268 | 1 | Village |
| Kell | 29 | 169 | 174 | -0.59% | 169 | 1 | Village |
| Vernon | 30 | 99 | 102 | -1% | 112 | 1 | Village |
| Walnut Hill | 31 | 92 | 96 | -1.08% | 242 | 0 | Village |
Vernon is the largest city in Marion County, with a population of 72,202.
Walnut Hill is the smallest city in Marion County, with a population of 92.
There are 31 cities in Marion County, Illinois.