15 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Milan | 1 | 1,731 | 1,821 | -1.03% | 976 | 2 | City |
| Milan | 2 | 1,731 | 1,818 | -1.03% | 976 | 2 | City |
| Green City | 3 | 563 | 596 | -1.23% | 393 | 1 | City |
| Green City | 4 | 563 | 595 | -1.23% | 393 | 1 | City |
| Green Castle | 5 | 206 | 219 | -1.44% | 461 | 0 | City |
| Green Castle | 6 | 206 | 219 | -1.44% | 461 | 0 | City |
| Newtown | 7 | 108 | 115 | -1.82% | 427 | 0 | Town |
| Newtown | 8 | 108 | 115 | -1.82% | 427 | 0 | Town |
| Humphreys | 9 | 87 | 91 | -1.14% | 349 | 0 | Village |
| Humphreys | 10 | 87 | 91 | -1.14% | 349 | 0 | Village |
| Harris | 11 | 58 | 61 | -1.69% | 377 | 0 | Town |
| Harris | 12 | 58 | 61 | -1.69% | 377 | 0 | Town |
| Pollock | 13 | 43 | 46 | -2.27% | 259 | 0 | Village |
| Pollock | 14 | 43 | 46 | -2.27% | 259 | 0 | Village |
| Winigan | 15 | 10 | 26 | 11.11% | 20 | 0 | CDP |
Milan is the largest city in Sullivan County, with a population of 1,731.
Winigan is the smallest city in Sullivan County, with a population of 10.
There are 15 cities in Sullivan County, Missouri.