Ecology, evolution and historical events all contribute to biogeographic patterns, but studies integrating them are scarce. Here we focus on how biotic exchanges of mammals during the Late Cenozoic have contributed to current geographic body size patterns. We explore differences in the environmental correlates and phylogenetic patterning of body size between the groups of mammals participating and not participating in past biotic exchanges. Both the association of body size with environmental predictors and its phylogenetic signal were stronger for groups that immigrated into North or South America than for indigenous groups. This pattern, which held when extinct clades were included in the analyses, can be interpreted based on the length of time that clades have had to diversify and occupy niche space. Moreover, we identify a role for historical events such as Cenozoic migrations on configuring contemporary mammal body size patterns and illustrate where these influences have been strongest for New World mammals.