This article adopts a mixed-methods approach to illustrate how economic, political, and social mechanisms work across time to shape individuals’ migration decisions. First, using large-scale survey data from the Mexican Migration Project, we show that economic, political, and social factors all matter for migration decisions. We nevertheless find evidence that social factors matter more for migration over time, net of economic and political considerations. Second, drawing on 120 in-depth interviews with migrants and their family members in four Mexican communities, we find that communities’ migration histories pattern how economic, political, and social factors contribute to migration decisions. In communities with limited migration histories, individuals report migrating in order to relieve economic pressures on themselves or their household members. In communities with more-established migration histories, a growing presence of current or returned migrants suggests the economic efficacy of a U.S. trip and helps to overcome potential difficulties to the journey. Finally, in communities with a high incidence of migration, social factors come to represent causes of migration—independent of financial need. Our findings provide a deeper understanding of the processes underlying Mexico-U.S. migration, which is crucial for anticipating future flows and crafting policy responses.