[3] Union County borders Colorado to the north, and Oklahoma and Texas to the east.

Union County, was created by an act of the territorial legislation in 1893 and was officially recognized on January 1, 1894, when the first slate of elected county officials received their oaths of office.

There were a number of bustling communities such as Amistad, Hayden, Sedan, Pasamonte, Gladstone, Mt Dora, Grenville, Des Moines, Folsom, and Dedman (now known as Capulin).

As of the 2000 census,[10] there were 4,174 people, 1,733 households, and 1,176 families living in the county.

30.00% of all households were made up of individuals, and 14.90% had someone living alone who was 65 years of age or older.

As of the 2010 census, there were 4,549 people, 1,695 households, and 1,073 families living in the county.

[11] In terms of ancestry, 13.6% were German, 10.3% were Irish, 7.4% were English, and 7.0% were American.

[14] Like all of Eastern New Mexico, Union County is heavily Republican.

Hillary Clinton in 2016 received the worst showing for a Democrat in the county's history – but this was partially due to a large migration towards third-party candidates such as New Mexico native and former governor Gary Johnson, who ran as a libertarian.