| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Mount Vernon | 1 | 14,091 | 14,566 | -0.68% | 962 | 15 | City |
| Ina | 2 | 1,602 | 1,648 | -0.62% | 666 | 2 | Village |
| Woodlawn | 3 | 607 | 621 | -0.65% | 712 | 1 | Village |
| Bluford | 4 | 603 | 623 | -0.66% | 380 | 2 | Village |
| Dix | 5 | 453 | 466 | -0.66% | 218 | 2 | Village |
| Waltonville | 6 | 403 | 415 | -0.74% | 325 | 1 | Village |
| Bonnie | 7 | 363 | 371 | -0.55% | 285 | 1 | Village |
| Belle Rive | 8 | 305 | 313 | -0.65% | 297 | 1 | Village |
| Nason | 9 | 196 | 201 | -1.01% | 215 | 1 | City |
Mount Vernon is the largest city in Jefferson County, with a population of 14,091.
Nason is the smallest city in Jefferson County, with a population of 196.
There are 9 cities in Jefferson County, Illinois.