Tana River mangabeys have a polygynous reproductive system, in which the females of a group typically only mate with the alpha male. Females have large estrous swellings every month for about four or five days, which signals to the male that she is fertile and receptive to copulation. Their mounting position is unique, with the male grasping the female’s ankles with his feet and placing his hands on her hips. Females also exhibit post-conception swelling. This may serve to mimic a female’s estrous swellings and trick males into believing she is fertile. This then confuses the paternity of the offspring and encourages paternal investment by all males with whom she mated. For example, if she conceives from copulation with the alpha male, her post-conception swellings may trick the beta male into believing she is fertile and mating with her. When she gives birth, both males have incentive to provide parental care, because they both mated with her around the time she conceived.

Gestation lasts 180 days, after which a single offspring is born. During the birth, other members of the group give plenty of space to the new mother, only approaching about 20 minutes after the offspring is born. More births tend to occur between August and April, the time of greatest food availability. Females give birth about every 18 to 24 months, and nearly two-thirds of adult females in a group give birth in a given year. Based on other species in their genus, Tana River mangabey females likely become sexually mature at about three years of age, breeding for the first time when they are six or seven. Males likely become sexually mature when they are about five, and breed for the first time when they are about seven. Little is known about parental care, although females tend to have the most sustained interest in young and likely provide most parental care.