This little-known species inhabits swamp forest and riverine habitat (Gautier 1995, Kingdon 1997, Malanda et al. 2004, Maisels et al. 2006, Gautier-Hion 2013). It usually forages on the ground and is omnivorous; animal matter comprises about 20% of the diet; it also eats fruit, seeds, young leaves and occasionally nectar from tall trees (Gautier-Hion and Maisels 1994). They have been reported to forage on crops. They sleep beside and over water, which is presumed to be an antipredator strategy. Groups consist of up to 57 individuals; one group of at least 85 has been seen (Gautier-Hion 2013). Females give birth to a single young.

Allen’s Swamp Monkey occurs throughout the swamp forests of the central Congolian basin in Democratic Republic of Congo (DRC) (both sides of the Congo River) and Republic of Congo (Gautier-Hion 2013). They are found throughout the Likouala-aux-Herbes swamps in Congo (Malanda et al. 2004); the most westerly records are from the upper Sangha River near the village of Bomassa in northern Congo (Maisels et al. 2006) and from a site north of Bayanga in Central African Republic (camera-trap image; Cassidy 2016; coupled with local hunters' knowledge: Remis and Jost Robinson 2012, Jost and Capel 2017). The Sangha forms the international border with Cameroon and it is therefore possible that the taxon occurs in Cameroon. Recent surveys in south-eastern Cameroon (Nzooh Dongmo et al. 2016) and southern CAR (N’Goran et al. 2016) do not mention the species, but swamp-dwellers are often overlooked by survey teams on foot (Maisels et al. 2006). Based on a combination of sightings, museum specimens and habitat availability, the known limits of its east-west distribution are from about 16°E to about 24°E, and its north-south distribution is from about 4°N to perhaps 6° S (Gautier 1985). Hart has searched intensively for this monkey but it seems not to occur east of 24° E.

This species is listed on Appendix II of CITES and in Class B of the African Convention on the Conservation of Nature and Natural Resources. Studies into the impact of hunting are needed.

The species occurs in the following protected areas: Salonga National Park and Lac Tumba-Ledima Forest Reserve (DRC), Dzanga-Sangha Special Reserve (CAR) and Lac Télé Community Reserve (Congo).