Tropical dry forests are among the world’s most imperiled biomes, and most long-lived and large-bodied animals that inhabit tropical dry forests persist in small, fragmented populations. Long-term monitoring is necessary for understanding the extent to which such populations can cope with changing climate conditions and recover after the elimination of human disturbances. We investigated how conservation measures, local rainfall patterns, and large-scale climate oscillations have affected the population dynamics of white-faced capuchins (Cebus capucinus) in a Costa Rican tropical dry forest over a 42-year period after the elimination of most human disturbances. The population’s rapid initial growth and later stabilization suggests that it was below the habitat’s carrying capacity at the time of the conservation area’s establishment. Management practices, such as aggressive fire suppression, may have played an important role in promoting this growth. Rainfall patterns were strongly coupled with phases and intensity conditions of the El Niño Southern Oscillation. The population experienced two distinct growth phases after the conservation area’s establishment, a period of rapid growth through the 1980s and 1990s and a subsequent period of stability from about 2000 to the present. El Niño-like conditions in the three years preceding a census year were associated with declines in reproductive output and/or offspring mortality during the rapid growth phase. The sensitivity of this ecosystem to global climatic phenomena suggests that some animals will be negatively affected if drought years become more common as the global climate warms.