The agile mangabey is a large, slender monkey, with relatively long limbs, a long muzzle, and a tail that is longer than its body. Males are much larger than females. The hair is short and its individual strands have different coloured bands down their length, giving the coat an overall brownish-grey appearance, which is darkest on the back, and paler fawn, with a white or yellowish tinge, on the underside, inner limbs and chin. In adults, the skin of the hands, feet, face and ears is black, and the tail is long, tapered and sometimes ends in a whitish tuft. The face has a white border and on the forehead a slight “whorl” of hair often exposes a pale patch of skin.

Mangabeys are quite vocal monkeys, the adult male agile mangabey producing a loud, long-distance call, a whoop followed by a rumble, which can be heard from up to a kilometre away, and may play a role in group spacing. Visual signals such as body postures and facial displays are also common, although the agile mangabey lacks the particularly pale eyelids which aid visual communication in other mangabey species.