Tropical forests are at the forefront of species-extinction crises as a consequence of widespread habitat loss and alteration. Knowledge of how populations respond to a particular milieu of habitat loss and disturbance effects requires long-standing monitoring programs. Unfortunately, such programs are rare. Here we report on population trends of de Brazza's monkey (Cercopithecus neglectus) over 20 years in Kenya's Kisere Forest, a biologically-rich moist forest remnant under intense anthropogenic pressure. We found that although abundance of this shy primate did not change across four censuses, group composition differed, with groups containing fewer juveniles after the first census. This result, in concert with a suite of recent empirical and predictive studies showing ongoing forest disturbance, suggests that we may see this population decline as it ages.