[2] The county is named after John Henninger Reagan (1818–1905), who was the postmaster general of the Confederate States and also a U.S. senator, U.S. representative, and first chairman of the Railroad Commission of Texas.

The region was first inhabited by Paleo-Indian, Suma-Jumano, Kiowa and Comanche peoples.

[3] Captains Hernán Martín and Diego del Castillo explored the region in 1650.

[4] In 1684, Juan Domínguez de Mendoza[5] and Nicolás López[6] reported on local indigenous groups.

In 1878, Camp Grierson's Spring was established as a subpost of Fort Concho and named in honor of Col. Benjamin H.

[10] The Kansas City, Mexico and Orient of Texas Railway was completed in 1911.

[13][14] In 1924, shortly after the oil boom, the town community of Best plunged into vice and violence, necessitating the intervention of the Texas Rangers.

[17] The Spraberry Trend, the third-largest oil field in the United States by remaining reserves, underlies much of the county.