11 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Aledo | 1 | 3,473 | 3,619 | -0.69% | 1,482 | 2 | City |
| Viola | 2 | 831 | 866 | -0.72% | 992 | 1 | Village |
| Alexis | 3 | 739 | 786 | -1.07% | 1,556 | 0 | Village |
| Sherrard | 4 | 686 | 691 | -0.15% | 739 | 1 | Village |
| Matherville | 5 | 681 | 707 | -0.58% | 1,742 | 0 | Village |
| New Boston | 6 | 600 | 614 | -0.33% | 638 | 1 | City |
| Keithsburg | 7 | 545 | 545 | 0% | 213 | 3 | City |
| Oak Grove | 8 | 469 | 471 | 0% | 713 | 1 | Village |
| Joy | 9 | 354 | 369 | -0.56% | 827 | 0 | Village |
| Seaton | 10 | 202 | 213 | -0.98% | 128 | 2 | Village |
| North Henderson | 11 | 157 | 162 | -0.63% | 695 | 0 | Village |
Aledo is the largest city in Mercer County, with a population of 3,473.
North Henderson is the smallest city in Mercer County, with a population of 157.
There are 11 cities in Mercer County, Illinois.