Reforming the UN Security Council seems a nearly impossible goal. Ongoing efforts since the early 1990s have stalled or dragged on for decades, with no clear resolution in sight. Only once—in 1963—has a reform initiative succeeded, expanding the non-permanent membership of the Council from 6 to 10 states, largely benefiting African and Asian countries. Why and how did UNSC reform prosper in 1963? This paper addresses this historical puzzle through primary research in the archives of six states: the United Kingdom, the United States, France, Mexico, Brazil, and Colombia. To draw comparative lessons, the paper also considers a subsequent reform initiative from the late 1970s and early 1980s, which ended in failure despite occurring in the same political context of the Cold War and decolonisation. The research demonstrates the importance of group cohesion and practical negotiating competence—both by the reformist Afro-Asian coalition in the 1960s and by the reluctant Great Powers in 1979-1981, and underscores the importance of procedural tactics and historical contingency in producing these outcomes.