A predominantly terrestrial species recorded from lowland to mid-montane tropical moist forest and montane bamboo forest. In the lowland forest zone (450–1,000 m), which comprises the major portion of the range, C. hamlyni occurs mainly in primary forest areas. The species occupies older secondary forest in the Okapi Wildlife Reserve, but does not favour young secondary forests, and is not known to occur in gallery forests in savannas at the southern limit of its range (Hart et al. 2013). The isolated forest blocks south of the species’ contiguous EOO, where C. hamlyni have been recorded historically, range in size from 200–450 km² and are located on small chains of hills. There is no evidence that these blocks were isolated due to historic human activities, and they may represent relictual range areas, isolated by climate-induced habitat change during the Quaternary or Holocene. In any case, they suggest that isolated populations of C. hamlyni may be able to persist even in a fragmented landscape. Further investigation of the current status of these populations, and exploration of other isolated forest massifs in this region for presence of the species is urgently needed.

Mean group size in the Kahuzi-Biega lowlands and Ituri Forest is 2–3 animals (Hall et al. 2002-2003), In Nyungwe National Park group size averaged 3.6 animals (Easton et al. 2011). These estimates are based on sightings of animals from line transects. However, given the difficulties of directly observing these monkeys, this may be an underestimate of group size. The species feeds primarily on the ground. Diet is poorly documented but known to include fruits and seeds. In the Ituri Forest (elevation 700–750 m) C. hamlyni forages opportunistically on sprouting seedlings when Gilbertiondendron dewevrei undergoes mast fruiting. Fungi and herbaceous vegetation are eaten during periods of fruit scarcity (Hart et al. 2013). The Nyungwe population feeds extensively on bamboo shoots, and this is likely to be the case in other montane locations such as Kahuzi-Biega and Mt. Tshiaberimu, where the species occurs in the bamboo zones. Given the range of habitats occupied, C. hamlyni appears to have a wide dietary tolerance with specialization based on local and seasonal food availability.

Leopards and crowned eagles are known to prey on C. hamlyni in the Ituri Forest; nonetheless, the species was recorded at a low frequency in one study of leopard diets (Hart et al. 1996).

The range of Cercopithecus hamlyni resembles that of Allochrocebus lhoesti, with a continuous EOO from the Congo/Lualaba River in the west, through the forested basins of the Lindi, Ituri, Maiko, Lowa, Ulindi and Elila rivers in eastern Democratic Republic of Congo (DRC), where the bulk of the range falls, into the submontane and montane forests of the Albertine Rift, where a small portion of the range is located. The range is bounded in the north by the Lindi-Nepoko river system and in the south by the southern limit of tropical moist forest (about 4° S). The only known population outside DRC occurs in an area of about 32 km² in Rwanda's Nyungwe National Park (Easton et al. 2011). A second Rwandan population in Gishwati is now likely extirpated due to habitat loss (Hart et al. 2013). Historically, populations of C. hamlyni have been reported from several small, isolated forest blocks, south to nearly 6° S (Hart et al. 2013); however, its current status in these areas is unknown. The eastern-most highland populations of C. hamlyni are now isolated in fragmented habitat islands, some of which have been recently extirpated, such as the forest islands between Kahuzi-Biega National Park and Masisi. The species has not been reported from areas of Burundi adjoining the Nyungwe population and its possible former occurrence in Uganda is not supported by museum specimens.

We estimate that the species’ elevational range extends from 450 m, where C. hamlyni has recently been recorded east of Kindu, 40 km from the Congo/Lualaba River (Hart pers. obs. 2018), to 3,200 m on Mt. Kahuzi and Mt. Tshiaberimu (Hart et al. 2013).

Cercopithecus hamlyni is listed on Appendix II of CITES and is included among DRC’s nationally protected species (Enerunga 2006); however, this law is largely unenforced outside protected areas. Given current trends in habitat loss, the single most important intervention for the in situ conservation of this species is guard patrols in national parks and other protected areas.

Owl-faced Monkeys occur in several protected areas with large blocks of intact habitat, including: Kahuzi-Biega and Maiko national parks, the Itombwe Massif Nature Reserve, the Okapi Wildlife Reserve and the Tayna Reserve. A large area in the Ituri Forest (Mai Tatu Forest) also harbours this monkey (Wildlife Conservation Society, unpublished data 2008, 2014), as does a small isolated forest block (North Balala) to the southeast of Itombwe (Wildlife Conservation Society, unpublished data 2014). It is found in small numbers at two sites in in Virunga National Park: Watalinga Forest in the north and Mt Tshiaberimu – a remnant patch of formerly contiguous habitat now covering only 75 km². Both of these populations, as well as the population in the upland sector of Kahuzi-Biega National Park, are isolated from other populations. The small population in Rwanda's Nyungwe National Park is also isolated. Additional research on population numbers and distribution is required, so as to better focus conservation efforts.