31 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Mercer | 1 | 3,870 | 3,956 | -0.44% | 109 | 35 | Township |
| Aledo | 2 | 3,536 | 3,618 | -0.45% | 1,509 | 2 | City |
| Richland Grove | 3 | 2,293 | 2,312 | -0.13% | 63 | 36 | Township |
| Preemption | 4 | 1,640 | 1,672 | -0.43% | 45 | 37 | Township |
| Greene | 5 | 1,425 | 1,447 | -0.28% | 39 | 37 | Township |
| New Boston | 6 | 1,161 | 1,173 | -0.26% | 23 | 51 | Township |
| Rivoli | 7 | 1,035 | 1,049 | -0.29% | 30 | 35 | Township |
| Coyne Center | 8 | 978 | 1,102 | 1.03% | 569 | 2 | CDP |
| Viola | 9 | 849 | 868 | -0.47% | 1,013 | 1 | Village |
| Alexis | 10 | 751 | 790 | -1.05% | 1,581 | 0 | Village |
| Matherville | 11 | 689 | 706 | -0.43% | 1,762 | 0 | Village |
| Sherrard | 12 | 688 | 693 | -0.15% | 741 | 1 | Village |
| Keithsburg | 13 | 640 | 640 | 0% | 36 | 18 | Township |
| Millersburg | 14 | 617 | 627 | -0.16% | 17 | 36 | Township |
| New Boston | 15 | 610 | 612 | 0% | 648 | 1 | City |
| Suez | 16 | 588 | 601 | -0.51% | 16 | 37 | Township |
| Keithsburg | 17 | 546 | 545 | 0% | 213 | 3 | City |
| Oak Grove | 18 | 460 | 469 | -0.43% | 699 | 1 | Village |
| Perryton | 19 | 444 | 455 | -0.67% | 12 | 36 | Township |
| Eliza | 20 | 405 | 410 | -0.25% | 7 | 57 | Township |
| North Henderson | 21 | 386 | 388 | 0% | 11 | 35 | Township |
| Abington | 22 | 365 | 372 | -0.27% | 10 | 37 | Township |
| Joy | 23 | 360 | 370 | -0.55% | 841 | 0 | Village |
| Viola | 24 | 301 | 305 | -0.33% | 9 | 35 | Township |
| Ohio Grove | 25 | 270 | 272 | 0% | 7 | 37 | Township |
| Duncan | 26 | 256 | 258 | 0% | 7 | 37 | Township |
| Seaton | 27 | 206 | 213 | -0.48% | 130 | 2 | Village |
| Preemption | 28 | 190 | 172 | 7.34% | 171 | 1 | CDP |
| North Henderson | 29 | 160 | 162 | 0% | 708 | 0 | Village |
| Cable | 30 | 131 | 103 | 7.38% | 115 | 1 | CDP |
| Millersburg | 31 | 78 | 76 | 8.33% | 263 | 0 | CDP |
Mercer is the largest city in Mercer County, with a population of 3,870.
Millersburg is the smallest city in Mercer County, with a population of 78.
There are 31 cities in Mercer County, Illinois.