15 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Aledo | 1 | 3,536 | 3,618 | -0.45% | 1,509 | 2.34 | City |
| Coyne Center | 2 | 978 | 1,102 | 1.03% | 569 | 1.72 | CDP |
| Viola | 3 | 849 | 868 | -0.47% | 1,013 | 0.84 | Village |
| Alexis | 4 | 751 | 790 | -1.05% | 1,581 | 0.47 | Village |
| Matherville | 5 | 689 | 706 | -0.43% | 1,762 | 0.39 | Village |
| Sherrard | 6 | 688 | 693 | -0.14% | 741 | 0.93 | Village |
| New Boston | 7 | 610 | 612 | 0% | 648 | 0.94 | City |
| Keithsburg | 8 | 546 | 545 | 0% | 213 | 2.56 | City |
| Oak Grove | 9 | 460 | 469 | -0.43% | 699 | 0.66 | Village |
| Joy | 10 | 360 | 370 | -0.55% | 841 | 0.43 | Village |
| Seaton | 11 | 206 | 213 | -0.48% | 130 | 1.58 | Village |
| Preemption | 12 | 190 | 172 | 7.34% | 171 | 1.11 | CDP |
| North Henderson | 13 | 160 | 162 | 0% | 708 | 0.23 | Village |
| Cable | 14 | 131 | 103 | 7.38% | 115 | 1.14 | CDP |
| Millersburg | 15 | 78 | 76 | 8.33% | 263 | 0.30 | CDP |
Aledo is the largest city in Mercer County, with a population of 3,536.
Millersburg is the smallest city in Mercer County.
There are 15 cities in Mercer County, Illinois.