27 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Mercer | 1 | 3,813 | 3,962 | -1.14% | 108 | 35 | Township |
| Aledo | 2 | 3,497 | 3,619 | -0.71% | 1,492 | 2 | City |
| Richland Grove | 3 | 2,252 | 2,308 | -0.84% | 62 | 36 | Township |
| Preemption | 4 | 1,622 | 1,676 | -1.04% | 44 | 37 | Township |
| Greene | 5 | 1,390 | 1,443 | -1.28% | 38 | 37 | Township |
| New Boston | 6 | 1,132 | 1,166 | -1.14% | 22 | 51 | Township |
| Rivoli | 7 | 1,012 | 1,049 | -1.27% | 29 | 35 | Township |
| Viola | 8 | 837 | 866 | -0.71% | 999 | 1 | Village |
| Alexis | 9 | 747 | 786 | -1.06% | 1,573 | 0 | Village |
| Sherrard | 10 | 687 | 691 | -0.15% | 740 | 1 | Village |
| Matherville | 11 | 685 | 707 | -0.72% | 1,752 | 0 | Village |
| Keithsburg | 12 | 637 | 642 | -0.47% | 35 | 18 | Township |
| Millersburg | 13 | 603 | 624 | -0.99% | 17 | 36 | Township |
| New Boston | 14 | 602 | 614 | -0.5% | 640 | 1 | City |
| Suez | 15 | 584 | 605 | -1.02% | 16 | 37 | Township |
| Keithsburg | 16 | 545 | 545 | 0% | 213 | 3 | City |
| Oak Grove | 17 | 469 | 471 | -0.21% | 713 | 1 | Village |
| Perryton | 18 | 438 | 456 | -1.35% | 12 | 36 | Township |
| Eliza | 19 | 397 | 409 | -1% | 7 | 57 | Township |
| North Henderson | 20 | 373 | 386 | -1.32% | 11 | 35 | Township |
| Abington | 21 | 357 | 372 | -1.38% | 10 | 37 | Township |
| Joy | 22 | 356 | 369 | -0.84% | 832 | 0 | Village |
| Viola | 23 | 301 | 305 | -0.33% | 9 | 35 | Township |
| Ohio Grove | 24 | 263 | 272 | -1.13% | 7 | 37 | Township |
| Duncan | 25 | 255 | 262 | -0.78% | 7 | 37 | Township |
| Seaton | 26 | 204 | 213 | -0.97% | 129 | 2 | Village |
| North Henderson | 27 | 158 | 162 | -0.63% | 699 | 0 | Village |
Mercer is the largest city in Mercer County, with a population of 3,813.
North Henderson is the smallest city in Mercer County, with a population of 158.
There are 27 cities in Mercer County, Illinois.