6 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Hannibal | 1 | 16,684 | 17,069 | -0.46% | 1,043 | 16 | City |
| Palmyra | 2 | 3,588 | 3,618 | -0.17% | 1,156 | 3.10 | City |
| Philadelphia | 3 | 378 | 186 | 7.39% | 499 | 0.76 | CDP |
| Rensselaer | 4 | 253 | 250 | 0.4% | 129 | 1.96 | Village |
| Sundown | 5 | 199 | 106 | 6.99% | 136 | 1.46 | CDP |
| Taylor township | 6 | 141 | 140 | 0% | 8 | 17.49 | Township |
Hannibal is the largest city in Marion County, with a population of 16,684.
Taylor township is the smallest city in Marion County.
There are 6 cities in Marion County, Missouri.