We present a study from Katavi National Park and surrounding areas that assessed the size and structure of the lion population as a baseline for wildlife management. We assessed lion and prey species density directly by sample surveys that incorporated specific detection probabilities. By using three prey-biomass regression models we also indirectly estimated lion density based on the assumption that these indirect estimates represent the Park's carrying capacity for lions. To identify key factors influencing lion abundance we conducted Spearman Rank correlation and logistic regression analyses, using prey species abundance and distance to Park boundary as explanatory variables. The mean size of the lion population was 31–45% of the estimated carrying capacity, with considerably fewer subadult males observed than expected. Lions generally avoided areas of up to 3 km from the Park boundary and were not observed outside the Park. Abundance of common prey species was significantly correlated with distance to the Park boundary and lion abundance. Lion abundance was most strongly associated with waterbuck abundance/presence. Based on observed lion demography, an evaluation of hunting quotas in adjacent hunting blocks, and anecdotal information on traditional lion hunting, we hypothesize that anthropogenic mortality of lions outside Katavi National Park is affecting lion abundance within the Park. Our results suggest that estimating lion densities with prey-biomass regression models overestimates densities even inside protected areas if these areas are subject to natural and anthropogenic edge effects.