| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Wellington | 1 | 7,512 | 7,678 | -0.38% | 986 | 8 | City |
| Belle Plaine | 2 | 1,436 | 1,473 | -0.42% | 1,730 | 1 | City |
| Conway Springs | 3 | 1,152 | 1,095 | 0.96% | 1,167 | 1 | City |
| Oxford | 4 | 1,036 | 1,047 | -0.19% | 1,143 | 1 | City |
| Caldwell | 5 | 994 | 1,027 | -0.6% | 920 | 1 | City |
| Argonia | 6 | 445 | 458 | -0.45% | 638 | 1 | City |
| South Haven | 7 | 328 | 328 | -0.3% | 328 | 1 | City |
| Geuda Springs | 8 | 158 | 154 | 0% | 459 | 0 | City |
| Mayfield | 9 | 71 | 71 | 0% | 240 | 0 | City |
| Milan | 10 | 48 | 48 | 0% | 364 | 0 | City |
| Hunnewell | 11 | 47 | 46 | 0% | 85 | 1 | City |
Wellington is the largest city in Sumner County, with a population of 7,512.
Hunnewell is the smallest city in Sumner County, with a population of 47.
There are 11 cities in Sumner County, Kansas.