18 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Monmouth township | 1 | 9,365 | 9,937 | -1.08% | 254 | 36.83 | Township |
| Monmouth | 2 | 8,347 | 8,896 | -1.3% | 1,968 | 4.24 | City |
| Roseville township | 3 | 1,100 | 1,167 | -0.99% | 30 | 36.64 | Township |
| Tompkins township | 4 | 832 | 892 | -1.3% | 23 | 36.55 | Township |
| Roseville | 5 | 831 | 890 | -1.42% | 1,124 | 0.74 | Village |
| Kirkwood | 6 | 629 | 676 | -1.41% | 670 | 0.94 | Village |
| Greenbush township | 7 | 507 | 536 | -0.98% | 14 | 35.53 | Township |
| Cameron | 8 | 414 | 146 | 7.25% | 654 | 0.63 | CDP |
| Coldbrook township | 9 | 412 | 436 | -0.96% | 12 | 35.57 | Township |
| Floyd township | 10 | 391 | 414 | -1.01% | 11 | 35.77 | Township |
| Kelly township | 11 | 349 | 370 | -0.85% | 10 | 35.14 | Township |
| Hale township | 12 | 306 | 330 | -1.61% | 8 | 36.54 | Township |
| Berwick township | 13 | 290 | 307 | -1.02% | 8 | 35.81 | Township |
| Ellison township | 14 | 287 | 313 | -2.05% | 8 | 36.40 | Township |
| Little York | 15 | 264 | 284 | -1.49% | 1,184 | 0.22 | Village |
| Lenox township | 16 | 254 | 270 | -1.17% | 7 | 36.34 | Township |
| Swan township | 17 | 226 | 241 | -1.31% | 6 | 35.31 | Township |
| Point Pleasant township | 18 | 129 | 136 | -0.77% | 4 | 36.02 | Township |
Monmouth township is the largest city in Warren County, with a population of 9,365.
Point Pleasant township is the smallest city in Warren County.
There are 18 cities in Warren County, Illinois.