Racialization and assimilation offer alternative perspectives on the position of immigrant-origin populations in American society. We question the adequacy of either perspective alone in the early twenty-first century, taking Mexican Americans as our case in point. Re-analysing the child sample of the Mexican American Study Project, we uncover substantial heterogeneity marked by vulnerability to racialization at one end but proximity to the mainstream at the other. This heterogeneity reflects important variations in how education, intermarriage, mixed ancestry and geographic mobility have intersected for Mexican immigrants and their descendants over the twentieth century, and in turn shaped their ethnic identity. Finally, based on US census findings, we give reason to think that internal heterogeneity is increasing in the twenty-first century. Together, these findings suggest that future studies of immigrant adaptation in America must do a better job of accounting for heterogeneity, not just between but also within immigrant-origin populations.