The interplay between density and climate in shaping the dynamics of herbivore populations is widely acknowledged, and current research is fueled by the identification of mechanisms underlying their effects on individuals and populations. We assessed whether forage availability mediated the effects of density and winter climate on body mass of white-tailed deer (Odocoileus virginianus) yearlings by experimentally reducing deer density to 7.5 and 15 deer/km2 during eight growing seasons, and by using causal (graphical) hierarchical models and Bayesian hierarchical modeling to assess relationships. The abundance of preferred forage decreased with deer density and varied quadratically (positive parabola) with winter North Atlantic Oscillation (NAO), whereas the fall mass of yearlings increased with forage abundance and spring mass. Fall mass did not differ between experimentally reduced deer densities, yet experimental yearlings were 30% heavier than yearlings harvested at ambient densities. Hence, forage abundance simultaneously mediated the effects of density and climate on fall body mass, which was also influenced by carry-over effects of spring body mass. Our findings increase our ability to anticipate how temperate large herbivores will respond to ongoing changes in intrinsic (e.g., large-herbivore density) and extrinsic (e.g., climate) factors.