Mule Deer is a medium-sized deer with a stocky body, slender legs, and a medium-length tail. Its common name stems from the characteristically large ears, which are light coloured on the inside with a dark brown-black rim. The coat is greyish brown in winter, and reddish brown in summer. The belly is the same colour as the rest of body, but the insides of the legs are lighter. There is a dark brown cap on the forehead extending from the ears to between the eyes; in older males, the cap is often darker along the sides just above the eyes and along the front edge. In adult males, this cap is especially noticeable because the face below it can be light grey to almost white. The cap usually does not reach the top of the eye, so there may be a lighter eye-ring above the eye. In summer coat, the cap is not always obvious. The nose is black and mainly hairless, sometimes with a narrow light or white area at the posterior edge, and the chin is also white with a vertical black stripe towards the back of the lips. There is always one white patch at the top of the neck below the throat, and sometimes a second one below it. The rump patch is white but varies in size with the subspecies, as does the colour of the tail. Mule Deer have poorly developed frontal skin glands located in the forehead. The metatarsal glands on the outside of the hind legs are large and well developed, as are the tarsal glands on the inner sides. Both pairs of these hind-leg glands are recognizable by the tufts of longer hair, sometimes of a darker colour, that mark their position. Other paired glands include antorbitals and interdigitals on each foot.
Young of the year are a reddish brown with white spots usually in horizontal rows along either side of the unbroken white mid-dorsal stripe. They also have a small light rump patch, and the hair inside the ears is light. At about 2.5 to 3 months of age, the coat becomes similar in colour to that of the adults.
The antlers of adult male Mule Deer show a bifurcated pattern: after the small brow tine, there are two pairs of relatively even forks, typically making a total of five tines on each antler. But atypical antlers are occasionally found on Rocky Mountain Mule Deer in B.C., and as many as 48 tines have been counted on the antlers of one animal. Atypical antlers are rarely found in either of the two Black-tailed subspecies. The antorbital depression (lachrymal pit) on the skull is relatively deep.