This mainly arboreal species is associated with riverine forest habitats. It is found close to rivers in lowland and submontane tropical moist forest, swamp forest, semi-deciduous forest and Acacia dominated forest. De Brazza’s Monkey is much more inconspicuous than most other guenons, with an extraordinary freezing antipredator behaviour, rarely using group calls and rarely found in polyspecific troops (Gautier-Hion and Gautier 1978). It lives in small family parties, at least in Gabon, northern Congo, the Batéké Plateau and central Cameroon (Maisels et al. 2007), although it can be found in groups of up to 35 individuals (Gautier-Hion 2013).

This widespread species ranges from north-eastern Angola, Cameroon, Equatorial Guinea (Rio Muni), Gabon, Congo and Central African Republic in the west of its range through DRC to Uganda, Kenya, South Sudan, and south-western Ethiopia in the east. It was formerly believed that the Sanaga River formed the northern boundary in Cameroon, but Maisels et al. (2007) extended the current known range for C. neglectus 100 km farther north in Cameroon, about 140 km farther south in Gabon, and about 80 km farther west in Congo than was previously known. The surveys were carried out in Cameroon in Mbam Djerem National Park, in Gabon in Batéké Plateau National Park, and the adjoining Batéké Plateau area in Congo. C. neglectus were found on both sides of the Djerem River in Cameroon, along the Mpassa and its tributaries in Gabon, along the Nambouli River in the Lefini Reserve in Congo, and up to the right bank of the Ogooué River on the Congo side of the border (Maisels et al. 2007). The most easterly limit of the species is the Mathews Range Forest Reserve of Samburu at an elevation of 2,200 m (the highest elevation at which the species has yet been recorded; Mwenja 2007, de Jong and Butynski 2010). In western Kenya, empirical and predictive studies showing ongoing forest disturbance, suggest that the population is declining (Walker and Sajita 2011).

This species has been listed on Appendix II of CITES since 1977 and on Class B of the African Convention on the Conservation of Nature and Natural Resources since 1968. It occurs in a number of protected areas across its range.