The Office of Strategic Services (OSS), an American intelligence agency during World War II, demonstrates the changing nature of anti-Catholicism within the American national security state during and after the war. Led by the Catholic lawyer, soldier, and politician William J. Donovan (1883−1959), the OSS eagerly sought to utilize Catholic individuals, ideas, and institutions as part of the war effort. The OSS worked with prominent Catholic leaders at home and abroad in an attempt to both guide Catholic perceptions of the war and leverage Catholic institutions as sources of strategic information. Somewhat paradoxically, the OSS and Donovan were interested in this information at least in part because of anti-Catholic assumptions then prevalent within the defense community and broader American culture. These anti-Catholic ideas shaped American intelligence operations during World War II and inadvertently increased demand for Catholic and other minority religious expertise within the post-war American intelligence system. The OSS’s success in working with Catholic institutions, including the Holy See, helped convince American defense and intelligence leaders of the strategic necessity of religious toleration and contributed to the growing pluralist discourse within the domestic United States.