Liberia has the largest blocks of continuous forest in West Africa, providing habitat for numerous wildlife species. However, there is a lack of empirical data about the status of Liberia's wildlife populations. During 2010–2012 we conducted the first nationwide survey in Liberia along c. 320 km of systematically located transect lines to estimate the abundance of chimpanzees Pan troglodytes verus, the diversity of large mammals and the nature and degree of anthropogenic threats. With > 7,000 chimpanzees, Liberia is home to the second largest population of West African chimpanzees and is therefore a priority for conservation of the species. Compared to the fragmented populations in other range countries the Liberian population is potentially one of the most viable. Our study revealed that the majority of chimpanzees and some of the most species-diverse mammal communities in Liberia exist outside protected areas. High hunting rates and plans for large-scale exploitation of natural resources necessitate rapid implementation of effective strategies to ensure the protection of one of West Africa's last strongholds for chimpanzees and other rare and threatened mammal species. We provide a country-wide baseline dataset that may serve as a platform for Liberian wildlife authorities, policy-makers and international conservation agencies to make informed decisions about the location and delineation of proposed protected areas, to identify conservation gaps and to devise a conservation action plan to conserve Liberia's wildlife resources.