| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Aledo | 1 | 3,483 | 3,618 | -0.66% | 1,486 | 2 | City |
| Viola | 2 | 830 | 867 | -0.72% | 990 | 1 | Village |
| Alexis | 3 | 748 | 783 | -0.93% | 1,575 | 0 | Village |
| Sherrard | 4 | 690 | 691 | -0.14% | 744 | 1 | Village |
| Matherville | 5 | 684 | 708 | -0.73% | 1,749 | 0 | Village |
| New Boston | 6 | 597 | 613 | -0.5% | 634 | 1 | City |
| Keithsburg | 7 | 545 | 545 | -0.18% | 213 | 3 | City |
| Oak Grove | 8 | 474 | 471 | -0.42% | 720 | 1 | Village |
| Joy | 9 | 354 | 369 | -0.84% | 827 | 0 | Village |
| Seaton | 10 | 206 | 216 | -0.96% | 130 | 2 | Village |
| North Henderson | 11 | 157 | 163 | -1.26% | 695 | 0 | Village |
Aledo is the largest city in Mercer County, with a population of 3,483.
North Henderson is the smallest city in Mercer County, with a population of 157.
There are 11 cities in Mercer County, Illinois.