Rapid population growth of reintroduced lions (Panthera leo) poses several ecological and management challenges in small (<1000 km²), fenced wildlife reserves. Changes in the natural social-ecological conditions of reintroduced lions can lead to a breakdown of natural predator–prey relationships and this is undesirable from both a biodiversity conservation and wildlife management perspective. Here, we present lion population vital rates across 16 small, fenced reserves in South Africa to test whether varying social ecological conditions, associated with lion population structures, can drive changes in vital rates. Study reserves were categorized (A–D) according to the number of resident prides and male coalitions. Lion population structure within a reserve affected lion vital rates differentially, and lion population growth was highest in reserves that contained a single resident pride. The presence of more than one pride in a reserve resulted in a younger age at first birth and marginally longer birth intervals. The presence of extra-pride adult males reduced cub survival, lioness age at first birth and birth intervals. These results can inform lion management decisions and, in particular, how lion population structure affects management outcomes. Our results also inform appropriate management protocols for lions depending on the specific social and ecological context of an individual reserve.