This largely arboreal species lives in the canopy of primary and old secondary lowland moist forest, and riverine and gallery forests. It is rare in degraded forest, but it appears that it can survive in lightly-logged forest where the canopy is relatively intact. This species occurs from just above sea level in Sierra Leone to at least 1,300 m asl in the Loma Mountains, Sierra Leone. Diana Monkeys feed mainly on fruits, arthropods and flowers; leaves become seasonally important when other items are less available (Buzzard 2006, Whitesides 1989). They spend much of their time in association with other primate species, which may be an anti-predator strategy (Whitesides 1991, Noë and Bshary 1997). Humans are the main predators on Diana Monkeys (Koné and Refisch 2007, McGraw 2007). Other important predators include Chimpanzees (Pan troglodytes), Leopards (Panthera pardus), and African Crowned Eagles (Stephanoatus coronatus) (Boesch and Boesch 1989, Shultz and Noë 2007, Oates 2011).

This species is found in the forests of Upper Guinea in West Africa, ranging from coastal southeastern Guinea to Sierra Leone, Liberia, and southern Côte d'Ivoire (west of the Sassandra River).

Cercopithecus diana is threatened by habitat loss and hunting pressure, and is listed on Appendix I of CITES and on Class A of the African Convention on the Conservation of Nature and Natural Resources.

The species is found in the following protected and semi-protected areas:

• Cote d'IvoireTai National Park (330,000 ha)
• Cavally Classified Forest (64,200 ha)
• Goin-Debe Classified Forest (133,100 ha)
• Sierra LeoneTiwai Island Wildlife Sanctuary (1,200 ha)
• Gola Forest National Park (71,070 ha)
• Loma Mountains National Park (33,200 ha)
• LiberiaSapo National Park (180,400 ha)