It is mainly olive or grey apart from the face (which is dark with a pale or yellowish patch on the forehead – the "diadem" from which the species derives its common name), the blackish cap, feet, and front legs, and the mantle, which is brown, olive, or grey depending on the subspecies.

It prefers to live in tall trees, which provide both food and shelter, and is, therefore, like almost all guenons, suffering from the loss of its natural habitat.

[8] Blue monkeys eat fruits, figs, insects, leaves, twigs, and flowers.

[10] Cercopithecus mitis joins with the C. ascanius (red-tailed monkey) for extra protection.

The blue monkeys live in female-philopatric social systems where females stay in their natal groups, while males disperse once they reach adulthood.

[3] As a result, blue monkey groups usually consist of one male with several females and infants, giving rise to matrilinear societies.

[16][21] Within-group conflicts are mild and infrequent because females distance themselves from one another and feed at different sites to avoid competition.

[16] The mating system is polygynous, with a corresponding sexual dimorphism in size, as the males are the substantially larger sex.

Females normally give birth every two years, during the onset of the warm, rainy season; gestation is around five months, and the infants are born with fur and with their eyes open.