Island biotas provide unparalleled opportunities to examine evolutionary processes. Founder effects and bottlenecks, for example, typically decrease genetic diversity in island populations, while selection for reduced dispersal can increase population structure. Given that support for these generalities mostly comes from single-species analyses, assemblage-level comparisons are needed to clarify how (i) colonization affects the gene pools of interacting insular organisms, and (ii) patterns of genetic differentiation vary within assemblages of organisms. Here, we use genome-wide sequence data from ultraconserved elements (UCEs) to compare genetic diversity and population structure of mainland and island populations of nine ant species in coastal southern California with respect to genetic diversity and population structure. As expected, island populations (from Santa Cruz Island) had lower than expected heterozygosity and Watterson’s theta compared to mainland populations (from the Lompoc Valley). Island populations, however, exhibited smaller genetic distances among samples, indicating less population subdivision and a higher capacity for dispersal compared to mainland populations. Within the focal assemblage, pairwise F_st values revealed pronounced interspecific variation in mainland-island differentiation, which increases with gyne body size. Our results reveal differences in genetic diversity and population genetic structure across an assemblage of interacting species, and illuminate general patterns of insularization in ants. Compared to single-species studies, our analysis of nine species pairs from the same island-mainland system offers a powerful approach to studying fundamental evolutionary processes.