27 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Mercer | 1 | 3,769 | 3,962 | -1.15% | 106 | 35 | Township |
| Aledo | 2 | 3,473 | 3,619 | -0.69% | 1,482 | 2 | City |
| Richland Grove | 3 | 2,233 | 2,308 | -0.84% | 61 | 36 | Township |
| Preemption | 4 | 1,605 | 1,676 | -1.05% | 44 | 37 | Township |
| Greene | 5 | 1,372 | 1,443 | -1.29% | 37 | 37 | Township |
| New Boston | 6 | 1,119 | 1,166 | -1.15% | 22 | 51 | Township |
| Rivoli | 7 | 999 | 1,049 | -1.28% | 29 | 35 | Township |
| Viola | 8 | 831 | 866 | -0.72% | 992 | 1 | Village |
| Alexis | 9 | 739 | 786 | -1.07% | 1,556 | 0 | Village |
| Sherrard | 10 | 686 | 691 | -0.15% | 739 | 1 | Village |
| Matherville | 11 | 681 | 707 | -0.58% | 1,742 | 0 | Village |
| Keithsburg | 12 | 634 | 642 | -0.47% | 35 | 18 | Township |
| New Boston | 13 | 600 | 614 | -0.33% | 638 | 1 | City |
| Millersburg | 14 | 597 | 624 | -1% | 16 | 36 | Township |
| Suez | 15 | 578 | 605 | -1.03% | 16 | 37 | Township |
| Keithsburg | 16 | 545 | 545 | 0% | 213 | 3 | City |
| Oak Grove | 17 | 469 | 471 | 0% | 713 | 1 | Village |
| Perryton | 18 | 432 | 456 | -1.37% | 12 | 36 | Township |
| Eliza | 19 | 393 | 409 | -1.01% | 7 | 57 | Township |
| North Henderson | 20 | 368 | 386 | -1.34% | 11 | 35 | Township |
| Joy | 21 | 354 | 369 | -0.56% | 827 | 0 | Village |
| Abington | 22 | 352 | 372 | -1.4% | 10 | 37 | Township |
| Viola | 23 | 300 | 305 | -0.33% | 8 | 35 | Township |
| Ohio Grove | 24 | 260 | 272 | -1.14% | 7 | 37 | Township |
| Duncan | 25 | 253 | 262 | -0.78% | 7 | 37 | Township |
| Seaton | 26 | 202 | 213 | -0.98% | 128 | 2 | Village |
| North Henderson | 27 | 157 | 162 | -0.63% | 695 | 0 | Village |
Mercer is the largest city in Mercer County, with a population of 3,769.
North Henderson is the smallest city in Mercer County, with a population of 157.
There are 27 cities in Mercer County, Illinois.