| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Monmouth | 1 | 8,502 | 8,923 | -1.13% | 2,004 | 4 | City |
| Roseville | 2 | 833 | 879 | -1.19% | 1,127 | 1 | Village |
| Kirkwood | 3 | 642 | 677 | -1.23% | 684 | 1 | Village |
| Little York | 4 | 263 | 281 | -1.5% | 1,179 | 0 | Village |
Monmouth is the largest city in Warren County, with a population of 8,502.
Little York is the smallest city in Warren County, with a population of 263.
There are 4 cities in Warren County, Illinois.