Eastern black-tailed rattlesnakes may be gray, olive or tan, with dark gray, brown or black rhombic patterns. They are variable in color and can be more yellow in higher elevations and more gray or brown in lower elevations. As the name suggests, this species has a distinct black tail often lacking any visible banding. Individuals also have a distinctive black "eyebrow" on their face.

The eastern black-tailed rattlesnake was recognized as a separate species distinct from the Black-tailed rattlesnake (Crotalus molossus ssp.) in 2012. The two species can best be distinguished by geographic range, occupying Texas, New Mexico, and northern Mexico in a variety of habitats from high conifer to dry, desert, alluvial slopes, arroyo and scrub regions. However, rock cover seems to be the most stable predictor of C. ornatus presence in any habitat.

Studies in western Texas have found that this species occupies home ranges of 0.72 - 60.3 hectares, with males occupying significantly larger home ranges than females. Both males and females occupy larger average home ranges than the closely related Crotalus molossus.

Its venom is primarily cytotoxic, with hemotoxic compounds. They prey primarily on rodents, especially the rock pocket mouse (Chaetodipus intermedius) and cactus mouse (Peromyscus eremicus). Eastern black-tailed rattlesnakes in temporary captivity were found to follow the scent cues of prey and set up ambush in sites with the heaviest traces of prey cues.