The anthropologist Oscar Lewis first used the term “culture of poverty” in a 1959 article on Mexico. Within months, the idea that the poor had a distinct culture became part of a passionate, decade-long, worldwide debate about poverty. Scholars, policy makers, and broader publics discussed what caused poverty and how to remedy it. How entrenched were the class and racial differences that led to poverty? How did those differences affect a country’s standing in the community of nations? This article tracks the concept of a culture of poverty as a way of probing the reciprocal, if unequal, connections between Mexico and the United States and their relation to national narratives and policy debates. It tracks how Lewis’s formulation of a culture of poverty drew on his training as an anthropologist in the United States, his extensive dialogue with Mexican intellectuals, and his fieldwork in Mexico. It also shows how Lewis and others reformulated the notion in response to intense public controversies in Mexico and Puerto Rico; the vehement U.S. discussions surrounding the War on Poverty and Daniel Patrick Moynihan’s report on the Negro family, and larger events such as the Cuban Revolution, the U.S. civil rights movement, decolonization, the Vietnam War, and second-wave feminism.