It is bounded on the west by Upton County, on the north by Glasscock County, on the east by Sterling, Tom Green, and Irion counties, and on the south by Crockett County. The county's northwestern corner lies on the Llano Estacado. Other towns include Best, Stiles, and Texon. Reagan County comprises 1,173 square miles of flat to gently sloping sandy terrain in the northwestern and north central regions and flat to sharply dissected limestone and rolling caliche in the remaining sections.
Soils are dark, calcareous stony clays and clay loams.
In the 1980s less than 1 percent of the land was considered prime farmland. Vegetation in the county consists of mesquite savanna, except in the southeastern corner where juniper, shinnery, and live oak also appear. Altitudes vary from 2,406 to 2,953 feet above sea level. The growing season extends for 229 days. Duststorms were once common during the spring, and the land suffered from overgrazing, water and wind erosion, and poor irrigation systems.