The male mona monkey has a head-and-body length of and a tail of . The female's head-and-body length is . The head has a brown crown, broad whitish brow band, grey mask of bare skin and bushy pale cheek fur. The upper parts of the body and the outer sides of the limbs are deep brown while the underparts and the insides of the limbs are creamy-white. On either side of the base of the tail are a pair of patches of white hair which closely resemble the ischial callosities found in baboons and other Old World monkeys in this position.
