In 1945, a vast range of US civic organizations and other groups were mobilized into a state-sanctioned campaign on behalf of a new international governance structure: the United Nations. This was a novel collaboration, one that demonstrated the State Department's acknowledgment of the value of civic activity and organized interests to securing foreign policy goals and that positioned US groups to assert an independent role in shaping the formal institutions of the United Nations. While scholars of American political development (APD) have tentatively embraced the notion that international institutions matter to American politics, past research on mid-twentieth century interests, conventionally focused on domestic business and trade associations, has underappreciated how and why the United Nations marked an important movement for interest development. Of particular significance, US voluntary and civic organizations were instrumental in securing a role for nongovernmental organizations in the UN Economic and Security Council, thereby further linking American and international politics and reshaping state-society relationships. In brief, this article argues that the State Department's campaign to mobilize public support around the United Nations, as well as the creation of the United Nations, generated new incentives for the maintenance and mobilization of existing groups and subsidized the formation of new groups.