| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Pella | 1 | 11,093 | 10,474 | 0.92% | 1,187 | 9 | City |
| Knoxville | 2 | 7,439 | 7,581 | -0.4% | 1,611 | 5 | City |
| Pleasantville | 3 | 1,700 | 1,687 | 0.06% | 695 | 2 | City |
| Melcher-Dallas | 4 | 1,194 | 1,198 | -0.17% | 1,148 | 1 | City |
| Bussey | 5 | 378 | 379 | -0.26% | 1,215 | 0 | City |
| Harvey | 6 | 247 | 238 | 0.41% | 365 | 1 | City |
| Hamilton | 7 | 125 | 120 | 0.81% | 247 | 1 | City |
| Swan | 8 | 79 | 76 | 0% | 128 | 1 | City |
| Marysville | 9 | 47 | 46 | 0% | 87 | 1 | City |
Pella is the largest city in Marion County, with a population of 11,093.
Marysville is the smallest city in Marion County, with a population of 47.
There are 9 cities in Marion County, Iowa.