It is one of two species assigned to the genus Mandrillus, along with the drill. Both the mandrill and the drill were once classified as baboons in the genus Papio, but they now have their own genus, Mandrillus. Although they look superficially like baboons, they are more closely related to Cercocebus mangabeys. Mandrills are found in southern Cameroon, Gabon, Equatorial Guinea, and Congo. Mandrills mostly live in tropical rainforests. They live in very large groups. Mandrills have an omnivorous diet consisting mostly of fruits and insects. Their mating season peaks in July to September, with a corresponding birth peak in December to April.
Mandrills are the world's largest monkeys. The mandrill is classified as vulnerable by IUCN.
The mandrill has an olive green or dark grey pelage with yellow and black bands and a white belly. Its hairless face has an elongated muzzle with distinctive characteristics, such as a red stripe down the middle and protruding blue ridges on the sides. It also has red nostrils and lips, a yellow beard and white tufts. The areas around the genitals and the anus are multi-colored, being red, pink, blue, scarlet, and purple. They also have pale pink ischial callosities. The coloration of the animal is more pronounced in dominant adult males. Both sexes have chest glands, which are used in olfactory communication. These, too, are more prominent in dominant adult males. Males also have longer canines than females, which can be up to 6.35 cm (2.50 in) and 1.0 cm, respectively.
The mandrill is one of the most sexually dimorphic mammals due to extremely strong sexual selection which favors males in both size and coloration. Males typically weigh 19–37 kg (42–82 lb), with an average mass of 32.3 kg (71 lb). Females weigh roughly half as much as the male, at 10–15 kg (22–33 lb) and an average of 12.4 kg (27 lb). Exceptionally large males can weigh up to 54 kg (119 lb), with unconfirmed reports of outsized mandrills weighing 60 kg (130 lb) per the Guinness Book of World Records. The mandrill is the heaviest living monkey, somewhat surpassing even the largest baboons such as chacma baboon and olive baboons in average weight even considering its more extreme sexual dimorphism, but the mandrill averages both shorter in the length and height at the shoulder than these species. The average male is 75–95 cm (30–37 in) long and the female is 55–66 cm (22–26 in), with the short tail adding another 5–10 cm (2–4 in). The shoulder height while on all fours can range from 45–50 cm (18–20 in) in females and 55–65 cm (22–26 in) in males. Compared to the largest baboons, the mandrill is more ape-like in structure, with a muscular and compact build, shorter, thicker limbs that are longer in the front and almost no tail. Mandrills can live up to 31 years in captivity. Females reach sexual maturity at about 3.5 years.