The first mass migration of Mexicans to the United States occurred in the early twentieth century: from smaller pre-Revolutionary flows in the 1900s, to hundreds of thousands during the violent 1910s, to the boom of the 1920s, and then the bust and deportations/repatriations of the 1930s. We show that despite these large shifts, the rate of economic assimilation was remarkably similar across arrival cohorts. We find that the average Mexican immigrant held a lower-paying job than US-born whites near arrival and further fell behind in the following decade. However, Mexican assimilation was not uniquely slow since we also find that the average Italian immigrant fell behind at a similar rate. Yet, conditional on geography, human capital, and initial earning score, Mexicans had a slower growth rate than both US-born whites and Italians. We argue that Mexican-specific structural barriers help to explain why Mexican progress was slower than other groups and why different Mexican arrival cohorts had limited variation in outcomes despite the large shocks to migration.