In the west of its range, this species is generally found in riparian or periodically-flooded swamp forest (Gautier-Hion and Brugière 2005). In Odzala-Kokoua National Park in Congo, they use seasonally-inundated and riverine habitats and occasionally the Marantaceae-dominated terra firma forests (Maisels 1995, 1996). In the area east of Makokou in Gabon, they were found in riverine forests (Quris 1975). By contrast, a study group at Mondika in CAR spent most of its time in terra firma mixed forest, and was never observed along the Mondika River, nor in swamp forest along the Ndoki River (Shah 2003), whereas a group at Bai Hokou, also in CAR, spent some time along rivers and streams (N. Shah pers. obs. 2003). Cercocebus agilis also uses open-understorey Gilbertiodendron dewevrei dominated forest (Shah 2003, Devreese 2011). In the north of its range, the species will use the gallery forests in savanna-forest mosaics of northern CAR (Fay 1988), and in the east (e.g., Okapi Wildlife Reserve), it uses terra firma forests, but will sometimes forage on crops (Kambale Saambere 1999). These monkeys are usually active in lower strata (0–10 m) of the habitat (Quris 1975, Shah 2003), although they spend less time on the ground than other Cercocebus spp. (Shah 2003). Home-range size was estimated to be ca 3 km² in Dzanga-Sangha in CAR (Shah 2003), and 2 km² in Gabon (Quris 1975).

Agile Mangabeys have strong dietary preferences for fruit, seeds and monocotyledon shoots, and can bite through the hard shells of some fruits, such as Strychnos (Shah 2003). They also prey on animals up to the size of infant Peters’ Duikers (Knights et al. 2008, Shah 2013).

In CAR, mean group size was 21 at Mondika, but a habituated group at nearby Bai Hokou numbered 135 animals, and on occasion it reached 330 individuals (Devreese et al. 2013). In northeastern Gabon, group size ranged between 8 and 18 (Quris 1975). In Odzala-Kokoua National Park (Congo), group size was usually >20 (Maisels 1996). In Ituri (DRC), it was 24 (Kambale Saambili 1999), in Ngotto Forest (CAR) it was 20 (D. Brugière, pers. comm. 2013). Groups of 50–200 have been observed in southeast Cameroon (Usongo and Fimbel 1995), northeast Gabon (Quris 1975) and northeast DRC (Kambale Saambili 1999).

The geographic range of Cercocebus agilis is not well known, despite covering about 1,010,000 km². This species occurs in Cameroon, Central African Republic (CAR), the Democratic Republic of Congo (DRC), northeast Gabon and northern Republic of Congo. It remains to be confirmed whether it still occurs in Equatorial Guinea.

The most westerly subpopulation, which may be isolated, appears to be in Campo Ma’an in southern Cameroon, as recorded by three different surveys (Etoga and Foguekem 2008, 2009; Nzooh Dongmo 2015). The Agile Mangabey is, therefore, expected to occur in Equatorial Guinea; however, since the 1960s, when a single adult female was recorded by Sabater Pi and Jones (1968), no survey has re-confirmed the presence of this species (Larison et al. 1999, Puit and Ghiurghi 2004, Sunderland 2005, Ruffler et al. 2012).

The northerly range limit is the forest ecotone from north of the Dja Biosphere Reserve in Cameroon, perhaps at 5°N, across to the Ubangui River, the gallery forest areas in CAR, up to about 9° N (Fay 1988). East of 22° E, the northern limit is likely around 6° 45’ N across to about 30°E, including the area of DRC southwards to at least the Tshopo River.

The southern limit is the right bank of the Congo River, probably from its confluence with the Tshopo (J. Hart pers. obs.) westward to the Sangha-Congo confluence at 1°6’ S. The range is then limited by the southwestern edge of the swamp forests (around 0°18’S up to just south of Odzala National Park (NP) at about 0°30’ S, and continues westwards across Gabon to Cameroon – west of the Dja Biosphere Reserve, near Sangemelima at around 11°30’ E (Gautier-Hion et al. 1999).

Cercocebus agilis is listed on Appendix II of CITES and on Class B of the African Convention on the Conservation of Nature and Natural Resources. Nationally, it is listed as partially protected in Cameroon, Congo and DRC. The hunting, sale, and consumption of primates are forbidden in Equatorial Guinea by Presidential Decree. It is on List C of Annex 2 to Ordinance 84.045 of 1984 in CAR, and not mentioned in Gabon’s protected species list. Cameroon, Gabon and Congo are currently revising their protected species lists.

To determine the western limits of its distribution, it is recommended that people familiar with both the call and appearance of C. torquatus, C. agilis (and Lophocebus albigena) undertake surveys in Campo Ma’an National Park and the areas between there and Dja Biosphere Reserve to be sure whether or not the two Cercocebus spp. are indeed sympatric. If C. agilis is confirmed there, then it may well also still occur in Equatorial Guinea and efforts should be made to look for this species there also.

Agile Mangabeys have been recorded in a number of protected areas; here we cite only the most recent survey information available:

Cameroon: Lobéké NP (Nzooh Dongmo et al. 2016a); Boumba Bek NP and Nki NP (Nzooh Dongmo et al. 2016b); Mengame Gorilla Sanctuary (Ekobo 2006); Dja Biosphere Reserve – already uncommon in 2009 (Latour 2010) and not recorded in 2015 (MINFOF/IUCN 2015; Campo Ma’an NP (Nzooh Dongmo 2015). Not recorded in Abong Mbam or Deng Deng in 2001 (Fotso et al. 2002), nor in Deng Deng in 2012 (Maisels et al. 2013a).

CAR: Dzanga-Ndoki NP (N’Goran et al. 2016), Mbaéré-Bodingué NP (Brugière et al. 2005); Zemongo Reserve (Roulet et al. 2007), Chinko Reserve (R. Hickisch & T. Aebischer, unpublished data 2016).

Congo: Nouabalé-Ndoki NP and Lac Télé Community Reserve (Brncic et al. 2018); Odzala-Kokoua NP (Maisels et al. 2013b).

DRC: Bili-Uéré Hunting Domain (Hicks 2014), Garamba NP (Hillman-Smith et al. 2014), Okapi Wildlife Reserve (Wildlife Conservation Society, unpublished data 2018).

Gabon: an early study of this species (Quris 1975) recorded its presence south of the Ivindo River (just to the east of what is now the north of Ivindo NP). C. agilis has not been recorded in protected areas of Gabon since. However, Shah (2013) mentions personal communications from well-informed scientists (L.J.T. White and S.A. Lahm) suggesting that Agile Mangabeys may still occur in Ivindo NP, Minkébé NP and Mwagna NP.