| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Milan | 1 | 1,724 | 1,818 | -1.03% | 972 | 2 | City |
| Green City | 2 | 563 | 600 | -1.23% | 393 | 1 | City |
| Green Castle | 3 | 209 | 219 | -0.95% | 468 | 0 | City |
| Newtown | 4 | 109 | 116 | -0.91% | 431 | 0 | Town |
| Humphreys | 5 | 87 | 92 | -1.14% | 349 | 0 | Village |
| Harris | 6 | 58 | 62 | -1.69% | 377 | 0 | Town |
| Pollock | 7 | 44 | 47 | -2.22% | 265 | 0 | Village |
Milan is the largest city in Sullivan County, with a population of 1,724.
Pollock is the smallest city in Sullivan County, with a population of 44.
There are 7 cities in Sullivan County, Missouri.