31 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Vernon | 1 | 70,580 | 71,290 | -0.21% | 1,970 | 35.83 | Township |
| Centralia | 2 | 13,753 | 14,317 | -0.66% | 392 | 35.08 | Township |
| Centralia | 3 | 11,645 | 12,152 | -0.86% | 1,420 | 8.20 | City |
| Salem | 4 | 8,704 | 9,075 | -0.7% | 246 | 35.40 | Township |
| Salem | 5 | 6,917 | 7,264 | -0.99% | 989 | 6.99 | City |
| Sandoval | 6 | 2,129 | 2,197 | -0.47% | 121 | 17.57 | Township |
| Odin | 7 | 1,525 | 1,578 | -0.52% | 85 | 18.01 | Township |
| Raccoon | 8 | 1,497 | 1,541 | -0.4% | 42 | 35.35 | Township |
| Stevenson | 9 | 1,132 | 1,181 | -0.7% | 31 | 36.66 | Township |
| Sandoval | 10 | 1,108 | 1,155 | -0.81% | 1,112 | 1 | Village |
| Kinmundy | 11 | 1,084 | 1,125 | -0.64% | 30 | 36.63 | Township |
| Iuka | 12 | 983 | 1,021 | -0.61% | 27 | 36.01 | Township |
| Tonti | 13 | 979 | 1,017 | -0.71% | 28 | 35.48 | Township |
| Salem | 14 | 909 | 942 | -0.33% | 25 | 35.78 | Township |
| Patoka | 15 | 894 | 933 | -0.78% | 26 | 34.94 | Township |
| Odin | 16 | 892 | 936 | -1% | 889 | 1 | Village |
| Haines | 17 | 845 | 868 | -0.35% | 23 | 36.06 | Township |
| Alma | 18 | 755 | 785 | -0.66% | 21 | 36.79 | Township |
| Kinmundy | 19 | 705 | 732 | -0.7% | 588 | 1.20 | City |
| Patoka | 20 | 498 | 523 | -0.99% | 447 | 1.11 | Village |
| Iuka | 21 | 484 | 508 | -1.02% | 660 | 0.73 | Village |
| Romine | 22 | 473 | 493 | -0.84% | 13 | 35.94 | Township |
| Omega | 23 | 428 | 447 | -0.7% | 12 | 35.42 | Township |
| Carrigan | 24 | 354 | 362 | 0% | 10 | 35.10 | Township |
| Foster | 25 | 340 | 362 | -1.16% | 10 | 35.56 | Township |
| Meacham | 26 | 334 | 350 | -0.6% | 9 | 36.53 | Township |
| Salem | 27 | 331 | 339 | -0.6% | 9 | 35.37 | Township |
| Alma | 28 | 299 | 314 | -0.99% | 264 | 1.13 | Village |
| Kell | 29 | 165 | 174 | -1.2% | 165 | 1 | Village |
| Vernon | 30 | 97 | 101 | -1.02% | 110 | 0.88 | Village |
| Walnut Hill | 31 | 91 | 96 | -1.09% | 239 | 0.38 | Village |
Vernon is the largest city in Marion County, with a population of 70,580.
Walnut Hill is the smallest city in Marion County, with a population of 91.
There are 31 cities in Marion County, Illinois.