Several generations of scholarship have identified the medieval development of urban self-government as crucial for European patterns of state formation. However, extant theories, emphasizing structural factors such as initial endowments and warfare, do little to explain the initial emergence of institutions of urban self-government before CE 1200 or why similar institutions did not emerge outside of Europe. We argue that a large-scale collapse of public authority in the ninth and tenth centuries allowed a bottom-up reform movement in West Francia (the Cluniac movement), directed by clergy but with popular backing, to push for ecclesiastical autonomy and asceticism in the eleventh and twelfth centuries. These social realignments, facilitated by new norms about ecclesiastical office holding, stimulated the urban associationalism that led to the initial emergence of autonomous town councils. Using a panel data set of 643 towns in the period between 800 and 1800, we show that medieval towns were substantially more likely to establish autonomous town councils in the period between 1000 and 1200 if they were situated in the vicinity of Cluniac monasteries. These findings are corroborated by regressions that use distance from Cluny—the movement's place of origin—to instrument for proximity to Cluniac monasteries.