4 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Monmouth | 1 | 8,500 | 8,916 | -0.81% | 2,004 | 4 | City |
| Roseville | 2 | 819 | 878 | -1.21% | 1,108 | 1 | Village |
| Kirkwood | 3 | 633 | 677 | -1.09% | 674 | 1 | Village |
| Little York | 4 | 261 | 281 | -1.14% | 1,170 | 0 | Village |
Monmouth is the largest city in Warren County, with a population of 8,500.
Little York is the smallest city in Warren County, with a population of 261.
There are 4 cities in Warren County, Illinois.