What kind of politics emerges under dire conditions of violence and precarity in the context of transit migration across Mexico? How is sanctuary practiced and gendered? This dissertation analyzes a range of experiences of transit and sanctuary practices of hospitality, care and solidarity, particularly those of marginalized women, nonbinary individuals, and grassroots movements that reveal an emerging pattern of transnational citizenship from below. As a defining issue of the 21st century, international migrations have provided an occasion for the rebirth of virulent forms of nationalist citizenship both from above and below, and the expansion of the coercive functions of the state. In the Americas, masses of displaced and dispossessed people cross international borders exposing themselves to the most perilous journeys throughout Mexico en routeto the United States. This dissertation sheds light on how civil society in Mexico has hastened to respond to the pressing needs of mobile populations, precipitated by a binational deportation regime that returns people to the poorest and most violent countries in the Western hemisphere. While the literature has privileged the (receiving) state as paradigmatic locus of power, and its institutional and geographical span as the main political domain, I argue for a notion of "citizenship in transit" that posits transit as a stark time-space within which the dispute to define the parameters and participants of citizenship unfolds. Taking "encounter" as a level of analysis, I shift the focus away from "foreigner" or "citizen" as identities and legal statuses, and direct attention instead to practice, to how people "do" citizenship. My study builds on data collected during fifteen months of ethnographic fieldwork involving participant observation and in-depth interviews with sanctuary workers, asylum seekers, undocumented migrants, volunteers, activists, lawyers, humanitarian personnel, and government officials in Mexico and Central America. My core argument is two-fold. In a material sense, transit becomes an enduring time-space wherein the multiplication of agents and sites of government undermine the rights of citizenship for everyone. As a set of political practices, "citizenship in transit" denotes how ordinary people re-invent citizenship beyond the legal designation and the epistemological framework of the nation-state.