Note: As defined by the US Census Bureau, a city's official boundaries often extend miles beyond the city itself and may even cross county lines. As such, several thousand US cities officially exist in two or more counties (and therefore appear on multiple counties' lists of cities.)
City | Rank | 2024 Pop. | 2020 Pop. | Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Aledo | 1 | 3,552 | 3,618 | -0.48% | 1,515 | 2.3 | City |
| Coyne Center | 2 | 968 | 1,102 | 1.04% | 563 | 1.7 | CDP |
| Viola | 3 | 853 | 868 | -0.47% | 1,018 | 0.8 | Village |
| Alexis | 4 | 759 | 790 | -1.04% | 1,598 | 0.5 | Village |
| Matherville | 5 | 692 | 706 | -0.57% | 1,770 | 0.4 | Village |
| Sherrard | 6 | 689 | 693 | -0.14% | 742 | 0.9 | Village |
| New Boston | 7 | 610 | 612 | -0.16% | 648 | 0.9 | City |
| Keithsburg | 8 | 546 | 545 | 0% | 213 | 2.6 | City |
| Oak Grove | 9 | 462 | 469 | -0.43% | 702 | 0.7 | Village |
| Joy | 10 | 362 | 370 | -0.55% | 846 | 0.4 | Village |
| Seaton | 11 | 207 | 213 | -0.96% | 131 | 1.6 | Village |
| Preemption | 12 | 177 | 172 | 7.27% | 159 | 1.1 | CDP |
| North Henderson | 13 | 160 | 162 | -0.62% | 708 | 0.2 | Village |
| Cable | 14 | 122 | 103 | 7.02% | 107 | 1.1 | CDP |
| Millersburg | 15 | 72 | 76 | 7.46% | 242 | 0.3 | CDP |
Note: City markers shown outside the county boundary mean that a part of the city extends into this county, but is primarily located in an adjacent county.
Aledo is the largest city in Mercer County.
Millersburg is the smallest city in Mercer County.
There are 15 cities in Mercer County, Illinois.