| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Clayton | 1 | 2,490 | 2,647 | -0.8% | 327 | 8 | Town |
| Des Moines | 2 | 103 | 114 | -1.9% | 118 | 1 | Village |
| Folsom | 3 | 45 | 49 | -2.17% | 69 | 1 | Village |
| Grenville | 4 | 23 | 25 | -4.17% | 61 | 0 | Village |
Clayton is the largest city in Union County, with a population of 2,490.
Grenville is the smallest city in Union County, with a population of 23.
There are 4 cities in Union County, New Mexico.