Historically, undocumented Mexican farm workers migrated circularly, leaving family behind in Mexico on short trips to the United States. Scholars have argued that border militarization has disrupted circular migration as the costs of crossing the border lead to longer stays, increased settlement, and changing transnational family practices. Yet, no study has explored changes in the transnational family structures of Mexico-U.S. migrants that span the era of border militarization. Using data from the National Agricultural Workers Survey, we document a dramatic shift away from transnational family life (as measured by location of residence of dependent children) among undocumented Mexican farm workers and a less dramatic shift among documented Mexican farm workers in the United States between 1993 and 2012. These trends are not explained by changes in the sociodemographic characteristics of farm workers or by changing demographic conditions or rising violence in Mexico. One-half of the trend can be accounted for by lengthened duration of stay and increased connections to the United States among the undocumented, but none of the trend is explained by these measures of settlement among the documented, suggesting that some Mexican farm workers adopt new family migration strategies at first migration. Increases in border control are associated with lower likelihood that children reside in Mexico-a finding that holds up to instrumental variable techniques. Our findings confirm the argument that U.S. border militarization-a policy designed to deter undocumented migration-is instead disrupting transnational family life between Mexico and the United States and, in doing so, is creating a permanent population of undocumented migrants and their children in the United States.