21 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Morgan | 1 | 1,375 | 1,431 | -1.43% | 27 | 51 | Township |
| Morgan | 2 | 1,375 | 1,420 | -1.43% | 27 | 51 | Township |
| Princeton | 3 | 978 | 1,008 | -0.51% | 615 | 2 | City |
| Princeton | 4 | 978 | 1,000 | -0.51% | 615 | 2 | City |
| Marion | 5 | 699 | 722 | -1.13% | 14 | 52 | Township |
| Marion | 6 | 699 | 717 | -1.13% | 14 | 52 | Township |
| Mercer | 7 | 258 | 264 | -0.39% | 701 | 0 | Town |
| Mercer | 8 | 258 | 262 | -0.39% | 701 | 0 | Town |
| Harrison | 9 | 207 | 213 | -0.96% | 5 | 44 | Township |
| Harrison | 10 | 207 | 212 | -0.96% | 5 | 44 | Township |
| Ravanna | 11 | 189 | 198 | -1.56% | 3 | 57 | Township |
| Ravanna | 12 | 189 | 197 | -1.56% | 3 | 57 | Township |
| Medicine | 13 | 130 | 139 | -2.99% | 3 | 49 | Township |
| Medicine | 14 | 130 | 138 | -2.99% | 3 | 49 | Township |
| Lindley | 15 | 109 | 112 | -0.91% | 2 | 58 | Township |
| Lindley | 16 | 109 | 111 | -0.91% | 2 | 58 | Township |
| Somerset | 17 | 87 | 90 | -1.14% | 2 | 44 | Township |
| Somerset | 18 | 87 | 90 | -1.14% | 2 | 44 | Township |
| Ravanna | 19 | 74 | 105 | 7.25% | 19 | 4 | CDP |
| South Lineville | 20 | 13 | 13 | 0% | 241 | 0 | Town |
| South Lineville | 21 | 13 | 13 | 0% | 241 | 0 | Town |
Morgan is the largest city in Mercer County, with a population of 1,375.
South Lineville is the smallest city in Mercer County, with a population of 13.
There are 21 cities in Mercer County, Missouri.