Understanding the biogeographic origins and temporal sequencing of groups within a region or of lineages within an ecosystem can yield important insights into evolutionary dynamics and ecological processes. Fifty years ago, Ernst Mayr generated comprehensive—if limited—inferences about the origins of the New World avifaunas, including the importance of pre-Isthmian dispersal between North and South America. Since then, methodological advances have improved our ability to address many of the same questions, but the phylogenies upon which such analyses should be based have been incompletely sampled or fragmentary. Here, we report a near-species-level phylogeny of the diverse (~832 species) New World clade Emberizoidea—the group that includes the familiar sparrows, cardinals, blackbirds, wood-warblers, tanagers, and their close relatives—to our knowledge the largest essentially complete (≥95%) phylogenetic hypothesis for any group of organisms. Biogeographic analyses based on this tree suggest initial dispersal into the New World via Beringia, with rapid subsequent diversification, including early dispersal of 1 lineage (the tanagers, Thraupidae) into South America. We found substantial dispersal between North and South America prior to closure of the Isthmus of Panama, but with a notable increase afterward, with a directional bias from north to south. With much greater detail and historical rigor, these analyses largely confirm Mayr's speculations based on taxonomy, resolving outstanding ambiguity regarding the continental origins of some groups such as the Emberizidae and Icteridae. The phylogeny reported here will be a resource of broad utility for addressing additional evolutionary and ecological questions with this diverse group.