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Across mammals, encephalization and longevity show a strong correlation. It is not clear, however, whether these traits evolved in a correlated fashion within mammalian orders, or when they do, whether one trait drives changes in the other. Here, we compared independent and correlated evolutionary models to identify instances of correlated evolution within six mammalian orders. In cases of correlated evolution, we subsequently examined transition patterns between small/large relative brain size and short/long lifespan. In four mammalian orders, these traits evolved independently. This may reflect constraints related to energy allocation, predation avoidance tactics, and reproductive strategies. Within both primates and rodents, and their parent clade Euarchontoglires, we found evidence for correlated evolution. In primates, transition patterns suggest relatively larger brains likely facilitated the evolution of long lifespans. Because larger brains prolong development and reduce fertility rates, they may be compensated for with longer lifespans. Furthermore, encephalization may enable cognitively‐complex strategies that reduce extrinsic mortality. Rodents show an inverse pattern of correlated evolution, whereby long lifespans appear to have facilitated the evolution of relatively larger brains. This may be because longer lived organisms have more to gain from investment in encephalization. Together, our results provide evidence for the correlated evolution of encephalization and longevity, but only in some mammalian orders. [ABSTRACT FROM AUTHOR]