Animals on islands often exhibit dramatic differences in morphology and behavior compared to mainland individuals, a phenomenon known as the "island syndrome". These differences, such as changes in body size and aggression, are thought to be adaptations to island environments, where there are high resource levels, low predation, limited dispersal, and thus high population densities. However, the extent to which island traits have a genetic basis or instead represent plastic responses to environmental extremes is often unknown. Here, we revisit a classic case of island syndrome in deer mice (Peromyscus maniculatus) from British Columbia. Previous field studies suggested that Saturna Island mice evolved several island traits, including higher body weight and reduced aggression relative to mainland populations. Using historical collections, we show that Saturna Island mice and those from neighboring islands are approximately 35% (~5g) heavier than mainland mice. We then collected mice from two focal populations: Saturna Island and a nearby mainland population. First, using molecular data, we find that these populations are genetically distinct, having diverged approximately 10 thousand years ago. Second, we established laboratory colonies and find that Saturna Island mice are heavier both because they are longer and have disproportionately more lean mass. These trait differences are maintained in second-generation captive-born mice raised in a common environment, implying a strong heritable component. In addition, island-mainland hybrids are heavier when born to island mothers than to mainland mothers, revealing a maternal genetic effect on body weight. Next, using behavioral testing in the lab, we also find that wild-caught island mice are less aggressive than mainland mice. However, lab-raised mice born to these founders do not differ in aggression, regardless of whether they are tested in conditions that induce low or high aggression, suggesting the large behavioral difference observed between wild-caught island and mainland individuals is likely a plastic response. Together, our results reveal that these mice respond differently to environmental conditions on islands, evolving both heritable changes in a morphological trait and also expressing a plastic phenotypic response in a behavioral trait.