25 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Mount Vernon | 1 | 13,958 | 14,570 | -0.73% | 953 | 15 | City |
| Mount Vernon | 2 | 11,988 | 12,516 | -0.71% | 324 | 37 | Township |
| Shiloh | 3 | 6,349 | 6,607 | -0.66% | 175 | 36 | Township |
| Dodds | 4 | 2,643 | 2,733 | -0.64% | 72 | 37 | Township |
| Spring Garden | 5 | 2,541 | 2,578 | -0.31% | 69 | 37 | Township |
| Webber | 6 | 2,064 | 2,141 | -0.63% | 56 | 37 | Township |
| Ina | 7 | 1,647 | 1,649 | 0% | 685 | 2 | Village |
| Field | 8 | 1,472 | 1,512 | -0.47% | 40 | 37 | Township |
| McClellan | 9 | 1,175 | 1,219 | -0.59% | 33 | 36 | Township |
| Casner | 10 | 1,131 | 1,170 | -0.62% | 31 | 36 | Township |
| Pendleton | 11 | 1,113 | 1,155 | -0.63% | 31 | 35 | Township |
| Grand Prairie | 12 | 877 | 902 | -0.34% | 24 | 36 | Township |
| Bald Hill | 13 | 765 | 807 | -1.03% | 21 | 36 | Township |
| Elk Prairie | 14 | 663 | 688 | -0.6% | 24 | 27 | Township |
| Dix | 15 | 637 | 623 | 1.11% | 12 | 54 | Township |
| Woodlawn | 16 | 600 | 620 | -0.5% | 704 | 1 | Village |
| Bluford | 17 | 594 | 623 | -0.83% | 375 | 2 | Village |
| Farrington | 18 | 564 | 577 | -0.18% | 15 | 37 | Township |
| Dix | 19 | 451 | 466 | -0.44% | 217 | 2 | Village |
| Waltonville | 20 | 395 | 416 | -0.75% | 319 | 1 | Village |
| Blissville | 21 | 369 | 379 | -0.54% | 11 | 35 | Township |
| Bonnie | 22 | 360 | 372 | -0.55% | 282 | 1 | Village |
| Moores Prairie | 23 | 346 | 360 | -0.57% | 10 | 36 | Township |
| Belle Rive | 24 | 296 | 308 | -0.67% | 288 | 1 | Village |
| Nason | 25 | 194 | 199 | -0.51% | 213 | 1 | City |
Mount Vernon is the largest city in Jefferson County, with a population of 13,958.
Nason is the smallest city in Jefferson County, with a population of 194.
There are 25 cities in Jefferson County, Illinois.