Understanding how ecological communities are organised and how they change through time is critical to predicting the effects of climate change. Studies on modern communities find that the shapes of body size distributions are weakly related to climatic variables and more strongly to habitat type, with flat distributions common in temperate habitats and peaked distributions common in tropical ones. In essence, increased habitat structure and productivity lead to more peaked body size distributions presumably because a greater number of ‘medium’ sized mammals can be supported. Because there have been major changes in mammalian community composition, body size, and global climate over the last 65 Ma, we ask how these patterns play out over geologic time. Using a database of Cenozoic mammal communities collected from the literature that spans multiple continents and habitat types, we analysed the shapes of community body size distributions and their relationships to habitat type and global climate. We find that 1) local body size distributions of Cenozoic mammals are weakly correlated with climate and more strongly with habitat type, 2) archaic and modern mammals show similar patterns in their body size distributions, and 3) maximum body size observed in local communities increases as mammals evolve larger body sizes and is correlated with climate change. The remarkable similarity in these patterns over the last 65 million years suggest a fundamental role of body size in community assembly, and that modern and archaic mammal faunas respond in similar ways to the environmental template.