We report the results of the first release project of 36 captive-bred mandrills into the Lékédi Park, Gabon. A mortality rate of 33% was recorded in the first year post-release, with dependent infants the most affected age class, as a result of environmental stress and malnutrition. A programme of provisioning ensured that individuals remained in a good physical condition. During the second year the death rate decreased to 4%, and 6-month survival rate of infants was 100%. Over time the mandrills extended their spatial use of the park, although their exploration remained limited. Our results demonstrate that provisioning and the lack of knowledge of ecological characteristics of the new, complex seasonal habitat were the likely causes of this situation. After 2 years, reproduction and survival appear stable. While habitat preservation and in situ species protection are the best conservation options, release projects may constitute a viable short-term solution for particular species. In this context, this first release of captive-bred mandrills could play a role in the future conservation of this Vulnerable species.