4 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Clayton | 1 | 2,514 | 2,649 | -0.87% | 330 | 8 | Town |
| Des Moines | 2 | 108 | 116 | -0.92% | 124 | 1 | Village |
| Folsom | 3 | 44 | 47 | 0% | 68 | 1 | Village |
| Grenville | 4 | 24 | 24 | 0% | 63 | 0 | Village |
Clayton is the largest city in Union County, with a population of 2,514.
Grenville is the smallest city in Union County, with a population of 24.
There are 4 cities in Union County, New Mexico.