It was named after James Wilmer Dallam, the lawyer who made the first digest of Texas laws.

[5] The first settlement in the area followed in 1870, which resulted in the Red River War of 1874 and 1875 with the native Comanche and Kiowa tribes.

In 1900–01, the Chicago, Rock Island and Pacific Railroad company built a stretch from Liberal, Kansas, to Tucumcari, New Mexico, which ran through the county.

[citation needed] The location where the tracks met those of the Fort Worth and Denver Railway was named Dalhart.

Within a short time, the small railroad stop turned into a sizable town and was named county seat in 1903.

[citation needed] Dallam County is located within District 86 of the Texas House of Representatives.

Joe Biden's 12.24% showing in the 2020 election is the lowest amount of the vote received by a Democrat since the county's organization in 1891.