9 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 |
| Ina | 2 | 1,647 | 1,649 | 0% | 685 | 2 | Village |
| Woodlawn | 3 | 600 | 620 | -0.5% | 704 | 1 | Village |
| Bluford | 4 | 594 | 623 | -0.83% | 375 | 2 | Village |
| Dix | 5 | 451 | 466 | -0.44% | 217 | 2 | Village |
| Waltonville | 6 | 395 | 416 | -0.75% | 319 | 1 | Village |
| Bonnie | 7 | 360 | 372 | -0.55% | 282 | 1 | Village |
| Belle Rive | 8 | 296 | 308 | -0.67% | 288 | 1 | Village |
| Nason | 9 | 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 9 cities in Jefferson County, Illinois.