As marine environments change, the greatest ecological shifts—including resource usage and species interactions—are likely to take place in or near regions of biogeographic and phylogeographic transition. However, our understanding of where these transitional regions exist depends on the defining criteria. Here we evaluate phylogeographic transitions using a bootstrapping procedure that allows us to focus on either the strongest genetic transitions between a pair of contiguous populations, versus evaluation of transitions inclusive of the entire overlap between two intraspecific genetic lineages. We compiled data for the Atlantic coast of the United States, and evaluate taxa with short- and long-dispersing larval phases separately. Our results are largely concordant with previous biogeographic and phylogeographic analyses, indicating strong biotic change associated with the regions near Cape Cod, the Delmarva Peninsula, and eastern Florida. However, inclusive analysis of the entire range of sympatry for intraspecific lineages suggests that broad regions—the Mid-Atlantic Bight and eastern Florida–already harbor divergent intraspecific lineages, suggesting the potential for ecological evaluation of resource use between these lineages. This study establishes baseline information for tracking how such patterns change as predicted environmental changes take place.