Although high rates of anthropogenic mortality are often reported for carnivores near reserve borders, the resulting edge effects are rarely quantified, and the consequences on conservation goals are usually unknown. Here, we assess the extent and impact of edge effects on a protected leopard Panthera pardus population in the Phinda-Mkhuze Complex (PMC), South Africa. We compared the spatial and demographic characteristics of leopards in two areas of the PMC, one closer to the border than the other. Leopard density declined from the core of the reserve (11.11 ± 1.31 leopards 100 km−2) to the border (7.17 ± 1.12 leopards 100 km−2), and was the lowest in non-protected areas adjoining the PMC (2.49 ± 0.87 leopards 100 km−2), but was not related to prey abundance or interspecific competition. Radiotelemetry showed that leopards near the border spent a greater proportion of their time outside the reserve and suffered higher annual mortality rates (0.358 ± 0.075) than those closer to the core (0.122 ± 0.065). A Cox proportional hazards model further demonstrated the negative effect of time spent outside the reserve on the survival probability of leopards. Despite an increased risk of mortality, leopards did not avoid non-protected areas, which may have functioned as an ecological trap for predators. Although the overall conservation outcome of the reserve was positive, edge effects clearly weakened the potential of the PMC to protect leopards. Our findings show that high mortality rates of carnivores in areas bordering reserves can extend to protected populations. Management approaches that control human activities on both sides of administrative borders are therefore essential if reserves are to conserve large carnivores effectively.