Global and project-level biodiversity indicators have received considerable attention, but indicators of the conservation actions and impacts of programmes and institutions appear to be under-developed. The IUCN Red List Index (RLI) has potential to be a useful indicator at an organizational-level to evaluate long-term impact of conservation on the extinction risk of species, thereby supporting institutional decision-making and communications. However, it has not yet been tested for its utility in tracking changes in extinction risk of a set of species targeted specifically by an individual conservation agency. Here, we examine the feasibility of using the RLI as one metric of the conservation impact of the Durrell Wildlife Conservation Trust, a conservation charity which runs multi-decadal programmes on a modest number of globally threatened terrestrial vertebrate species. Of 17 target amphibian, bird and mammal species, eight underwent improvements in Red List category (reductions in extinction risk) owing to conservation. This drove a 67% increase in the value of the Red List Index between 1988 and 2012. This contrasts with a 23% decline in a counterfactual RLI showing projected trends if conservation had been withdrawn in 1988. For organizations that target sets of species with circumscribed geographic distributions and that are regularly assessed by the IUCN Red List, the RLI is a useful indicator for measuring and demonstrating long-term conservation impact to technical and non-technical audiences.