The mantled guereza is diurnal and arboreal, found in both deciduous and evergreen forests. It is an adaptable species that can cope with habitat disturbance and prefers secondary forest close to rivers or lakes. Although previously thought only to eat leaves, it also eats seeds, fruits, and arthropods. It is able to digest plant material with a high fibre content with its specialised stomach and may only eat from a few plant species at a time. It is preyed on by birds of prey and some mammals, such as the common chimpanzee and the leopard.
The mantled guereza lives in social groups of three to fifteen individuals. These groups normally include a dominant male, several females, and the offspring of the females. It has a polygynous mating system and copulation is initiated with vocal communication. After a gestation period of just over five months, infants are born with pink skin and white fur, which darkens to the adult coloration by three to four months. The mantled guereza is well known for its dawn chorus, the males' "roar" is a method of long distance communication that reinforces territorial boundaries. It also makes other vocalization and uses body postures, movements, and facial expressions to communicate. The mantled guereza is listed as Least Concern by the International Union for Conservation of Nature (IUCN) because it is widespread - although it is locally threatened in some areas, the decline is not great enough to list it in a higher category of threat. However, one subspecies found in Kenya is listed as Endangered. It can survive well in degraded forests and in some areas it is more common in logged areas than unlogged ones. The mantled guereza is also threatened by hunting for bushmeat and for its skin.
The mantled guereza has a distinctive pelage, it is mostly black, with long white fringes of silky hair—known as a mantle or ornamentation—along the sides of its body and tail. The bands that make up the mantle start at the shoulders and extend along the back until they connect at the lower torso. The tail is long and ends in a white tuft which varies in how much it covers the tail. These features vary in color among subspecies, for example the tail of C. g. guereza is gray until the white tail tuft which covers half of its length, while the tail tuft of C. g. caudatus makes up 80% of the tail. The mantle color ranges from white to cream or yellow. Its face is framed by white hair and it has bushy cheek hairs. The thigh has a white stripe. Infants are born with pink skin and white hair. The hair and skin darken as they age and by three to four months they attain adult coloration. Male usually gain their coloration before females. The male typically weighs 9.3 and 13.5 kilograms (21 and 30 lb) and the female weighs between 7.8 and 9.2 kilograms (17 and 20 lb). The head and body length averages 61.5 centimetres (24.2 in) for males and 57.6 centimetres (22.7 in) for females. Like most colobi, the mantled guereza has a small thumb that is vestigial. There is dentition sexual dimorphism among the subspecies. In some, the males have larger teeth than females, in others the reverse is true, and some have no significant difference.
The mantled guereza is distributed throughout Equatorial Africa. It ranges from Nigeria and Cameroon in the west to Ethiopia, Kenya, Uganda, and northern Tanzania in the east. The mantled guereza lives in both deciduous and evergreen forests. It mainly inhabits forest and savannah woodlands and often extend into highland and montane forests. It can be found in other forest habitats, both primary and secondary, such as riparian (near fresh or brackish water), gallery, and upland forests. It is particularly common in forests close to rivers and lakes and at high elevations. It can be found in elevations as high as 3,300 metres (10,800 ft). This species prefers secondary forests and selects them over old-growth forests if given the choice. It is likely that the mantled guereza prefers these forests due to increased the number of food trees and the weaker chemical defenses of the species within. The mantled guereza is sometimes found in swamps as well as human-made habitats such as Eucalyptus plantations, which may be frequented when the monkey has nutritional deficiencies.
The mantled guereza is primarily arboreal, but does sometimes descend on the ground to forage and travel, perhaps more so than most other colobines. It is diurnal and rests for up to half the day. Foraging or travelling are the next most common activity. Sometime after dawn, mantled guereza groups leave their sleeping trees and will return to them at dusk. During the day, the mantled guereza has long rest periods in between periods of moving and feeding. Other activities, including grooming, greeting, playing and being vigilant, are performed to a lesser extent. Despite its reputation as an exclusive leaf-eater, the mantled guereza is not an obligate folivore. While it mainly eats leaves and fruit, its diet is quite variable. It may eat bark, wood, seeds, flowers, petioles, lianas, aquatic-plants, arthropods, soil, and even concrete from buildings. The amount of each food item in its diet varies by area and time of year. Nutritional factors like protein, tannins, and sodium levels in leaves influence its food choices. It may even intermittently travel longer distances to access plants with higher levels of nutrition. Leaves usually make up over half of its diet, although fruits are occasionally eaten more depending on the season. When foraging for leaves, the mantled guereza prefers young ones over old. With fleshy fruits, the mantled guereza prefers to eat them unripe, which may serve to reduce competition with primates that eat ripe fruits. It consumes a number of plant species but only some make up most of its diet at a specific site.
Like all colobi, the mantled guereza is able to digest leaves and other plant fibers with a large, multi-chambered stomach that contains bacteria in certain areas. Like most colobines, it prefers foods with high fiber content that can be easily extracted with its specialized stomach. The mantled guereza is mostly preyed on by the crowned hawk-eagle, but it is also eaten by other birds of prey such as Verreaux's eagle. The common chimpanzee is known to hunt the guereza. The leopard is another possible predator.
The mantled guereza lives in stable social groups usually containing three to fifteen members. The groups usually contain one male, several females and juveniles. In some populations, groups containing several males are common. In multi-male groups, males tend to be aggressive with one another with one being dominant. Some males may be expelled from these groups. Multi-male groups may contain father-son pairs or unrelated males. Males that are not part of groups either live solitarily or with other outside males in bachelor groups. The females keep the groups cohesive and they are matrilineally related. They rarely disperse from their natal groups, except possibly when they break apart. Males on the other hand, usually leave when they become subadults or adults. They may start out being solitary and or in bachelor groups. They gain entry into a social group either by being on the periphery or displacing a group male.
Because of its low quality diet and the dispersed distribution of its food, the mantled guereza has a resident-egalitarian social structure. Female guerezas living in a group often have an egalitarian dominance style with no formalized rank relations. Relationships are relaxed and friendly with rare signals of dominance or subordinance. Physical aggression within the group is usually not harmful and rarely escalates into a conflict. Allogrooming is an important part of mantled guereza interactions and mostly occurs between females. The adult males rarely groom in the groups. While not strictly territorial, mantled guereza groups can be aggressive towards each other. In some populations, groups may defend core areas (which exist as a small part of the home range), resources, and mates. During intergroup encounters, males can engage in direct or indirect mate defense, like defending a female's resources. It is the males that participate in agonistic inter-group encounters but female may do so as well. Aggressive encounters between groups usually involve chases, displays and vocalizations rather than physical contact.