To what extent is the work of international organizations shaped by their most powerful members? Can minor powers influence the decisions taken at these organizations? This paper presents the argument that great powers engage in power-sharing in order to attain unanimity inside international organizations, which enhances compliance and reduces the cost of implementing their decisions. An analysis of the UN Security Council tests this argument. Challenging the conventional wisdom that minor powers' influence on the Council is negligible, this paper identifies a series of informal power-sharing practices, which promote consensus and augment minor powers' influence far beyond what one would expect on the basis of the material capabilities and formal voting power of these states. The study relies on a novel design-based approach, which exploits exogenous variation in Africa's participation on the Security Council to estimate the influence of African states inside this body. Non-parametric permutation tests and a qualitative case study show that African states have a substantial impact on the Council's response to civil wars in Africa between 1988 and 2014. During years when a given African region is represented on the Security Council, the UN deploys an average of 920 more peacekeepers and allocates larger peacekeeping budgets to civil-war countries in that region than during years without a member of the Council from that region. This effect of a seat on the Council is particularly pronounced during major crises, when great powers are most eager to attain unanimity through power-sharing, and while minor powers benefit from the informal authority of the Council's rotating presidency. Informal power-sharing inside international organizations such as the Security Council, motivated by the self-interests of powerful states, enhances the influence of minor powers.