18 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Monmouth | 1 | 9,365 | 9,937 | -1.08% | 254 | 37 | Township |
| Monmouth | 2 | 8,347 | 8,896 | -1.3% | 1,968 | 4 | City |
| Roseville | 3 | 1,100 | 1,167 | -0.99% | 30 | 37 | Township |
| Tompkins | 4 | 832 | 892 | -1.3% | 23 | 37 | Township |
| Roseville | 5 | 831 | 890 | -1.42% | 1,124 | 1 | Village |
| Kirkwood | 6 | 629 | 676 | -1.41% | 670 | 1 | Village |
| Greenbush | 7 | 507 | 536 | -0.98% | 14 | 36 | Township |
| Cameron | 8 | 414 | 146 | 7.25% | 654 | 1 | CDP |
| Coldbrook | 9 | 412 | 436 | -0.96% | 12 | 36 | Township |
| Floyd | 10 | 391 | 414 | -1.01% | 11 | 36 | Township |
| Kelly | 11 | 349 | 370 | -0.85% | 10 | 35 | Township |
| Hale | 12 | 306 | 330 | -1.61% | 8 | 37 | Township |
| Berwick | 13 | 290 | 307 | -1.02% | 8 | 36 | Township |
| Ellison | 14 | 287 | 313 | -2.05% | 8 | 36 | Township |
| Little York | 15 | 264 | 284 | -1.49% | 1,184 | 0 | Village |
| Lenox | 16 | 254 | 270 | -1.17% | 7 | 36 | Township |
| Swan | 17 | 226 | 241 | -1.31% | 6 | 35 | Township |
| Point Pleasant | 18 | 129 | 136 | -0.77% | 4 | 36 | Township |
Monmouth is the largest city in Warren County, with a population of 9,365.
Point Pleasant is the smallest city in Warren County, with a population of 129.
There are 18 cities in Warren County, Illinois.