Global biodiversity loss is occurring at an alarming rate, with a growing consensus that we are entering a sixth mass extinction. Rapid human development and associated habitat loss, principally through agriculture, infrastructure and extractive industries, is a significant contributor. The critically endangered western chimpanzee (Pan troglodytes verus) is at the nexus of the development-habitat loss conflict with 83% of the subspecies occurring outside protected areas. To protect its habitat, robust regulation of large-scale development, in particular through Environmental Impact Assessment (EIA) procedures that adequately incorporate the Mitigation Hierarchy Principles (MHPs), is required. We developed a framework to review EIA and related legislation in all eight western chimpanzee range countries and assessed how well they incorporate MHPs, focusing specifically on mining activity. Costa Rica was selected as a benchmark of good practice, based on the recognised quality of its biodiversity laws. Overall performance of the eight countries fell substantially below that of Costa Rica. Whilst isolated good examples were identified, the overall approach legislates for broad concepts but fails to deliver on detailed provisions. As recommended by the current regional action plan for the conservation of western chimpanzees, all countries would benefit from urgent legislative reform to better incorporate MHPs within EIA procedures. This study provides recommendations in that regard together with a replicable tool for assessing EIA legislation in other jurisdictions. •Western chimpanzees are threatened by a lack of regulation of development, particularly mining.•Mitigation hierarchy principles are critical to species conservation.•The mitigation hierarchy is not uniformly incorporated into environmental impact assessment and related laws.•Offset measures are all but absent from the legislation of the western chimpanzee range countries.•Environmental impact assessment laws require reform to embed mitigation hierarchy principles.