The practice of city planning in New York City was transformed in the decades after World War II. At the start of this period, the system was characterized by little citizen involvement and no transparency. By the mid-1970s, citizens had become accepted participants in land-use decision-making, and formal procedures for involving citizens in planning had been written into local law. This article explores how this turning point in citizen participation came about by focusing on the Cooper Square Committee—an ambitious practitioner of neighborhood activism on the Lower East Side of Manhattan. Setting the Committee’s quest in the participatory context of the times uncovers a groundswell of voluntary groups who used the city’s neighborhoods as forums for democratic action. Along with government actors, planning professionals, and civic and social agencies, such groups contributed to the transformation in planning, which developed not by premeditated campaign but by a cumulative process of public problem-solving and social innovation.