11 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Morgan | 1 | 1,407 | 1,423 | 0.07% | 27 | 51 | Township |
| Princeton | 2 | 980 | 1,000 | -0.41% | 617 | 2 | City |
| Marion | 3 | 707 | 715 | 0% | 14 | 52 | Township |
| Mercer | 4 | 257 | 262 | -0.39% | 698 | 0 | Town |
| Harrison | 5 | 211 | 214 | 0% | 5 | 44 | Township |
| Ravanna | 6 | 189 | 195 | -0.53% | 3 | 57 | Township |
| Medicine | 7 | 141 | 140 | 0.71% | 3 | 49 | Township |
| Lindley | 8 | 109 | 111 | 0% | 2 | 58 | Township |
| Somerset | 9 | 90 | 92 | 0% | 2 | 44 | Township |
| Ravanna | 10 | 74 | 105 | 7.25% | 19 | 4 | CDP |
| South Lineville | 11 | 13 | 13 | 0% | 241 | 0 | Town |
Morgan is the largest city in Mercer County, with a population of 1,407.
South Lineville is the smallest city in Mercer County, with a population of 13.
There are 11 cities in Mercer County, Missouri.