26 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Mount Vernon | 1 | 14,060 | 14,570 | -0.72% | 960 | 15 | City |
| Mount Vernon | 2 | 12,074 | 12,516 | -0.71% | 326 | 37 | Township |
| Shiloh | 3 | 6,391 | 6,607 | -0.65% | 176 | 36 | Township |
| Dodds | 4 | 2,660 | 2,733 | -0.64% | 72 | 37 | Township |
| Spring Garden | 5 | 2,549 | 2,578 | -0.31% | 69 | 37 | Township |
| Webber | 6 | 2,077 | 2,141 | -0.62% | 56 | 37 | Township |
| Ina | 7 | 1,647 | 1,649 | -0.06% | 685 | 2 | Village |
| Field | 8 | 1,479 | 1,512 | -0.47% | 40 | 37 | Township |
| McClellan | 9 | 1,182 | 1,219 | -0.59% | 33 | 36 | Township |
| Casner | 10 | 1,138 | 1,170 | -0.61% | 32 | 36 | Township |
| Pendleton | 11 | 1,120 | 1,155 | -0.62% | 32 | 35 | Township |
| Grand Prairie | 12 | 880 | 902 | -0.34% | 24 | 36 | Township |
| Bald Hill | 13 | 773 | 807 | -1.02% | 22 | 36 | Township |
| Elk Prairie | 14 | 667 | 688 | -0.6% | 24 | 27 | Township |
| Dix | 15 | 630 | 623 | 1.12% | 12 | 54 | Township |
| Woodlawn | 16 | 603 | 620 | -0.66% | 708 | 1 | Village |
| Bluford | 17 | 599 | 623 | -0.83% | 378 | 2 | Village |
| Farrington | 18 | 565 | 577 | -0.18% | 15 | 37 | Township |
| Dix | 19 | 453 | 466 | -0.66% | 218 | 2 | Village |
| Waltonville | 20 | 398 | 416 | -1% | 321 | 1 | Village |
| Blissville | 21 | 371 | 379 | -0.54% | 11 | 35 | Township |
| Bonnie | 22 | 362 | 372 | -0.55% | 284 | 1 | Village |
| Moores Prairie | 23 | 348 | 360 | -0.57% | 10 | 36 | Township |
| Belle Rive | 24 | 298 | 308 | -0.67% | 290 | 1 | Village |
| Opdyke | 25 | 260 | 303 | 0% | 492 | 1 | CDP |
| Nason | 26 | 195 | 199 | -0.51% | 214 | 1 | City |
Mount Vernon is the largest city in Jefferson County, with a population of 14,060.
Nason is the smallest city in Jefferson County, with a population of 195.
There are 26 cities in Jefferson County, Illinois.