This chapter analyzes the institutional design phase and early years of the United Nations (UN) Trusteeship System with reference to its predecessor the League of Nations Mandate System, the drafting stages of its provisions and objectives, as well as by establishing its relevance for intergovernmental organizations (IGOs) accountability and oversight; here as a platform for anti-colonial actors bend to advance decolonization. To be clear, the institutional design of the Trusteeship System furthered the interests of the great powers, in particular the United States and Great Britain, nonetheless, during its setup, it became imbued with objectives and procedural innovations anti-colonial actors could draw on to advance decolonization through the UN. By making this argument, this chapter makes two larger claims: First, and more broadly, an institutional design phase in an IGO offers unique, one-off opportunities for a variety of actors to shape global governance institutions by establishing critical pathways for change. Especially the roles of anti-colonial actors in and around the UN’s institutional design secured limited advances to access and participate in the IGO while keeping colonial states from backsliding. Second, the setup of the Trusteeship Council – with its unique rules and unprecedented procedures – created an accountability venue that, despite holding colonial interests intact, proved to establish a wedge in the colonial system. The Trusteeship Council, although limited in its efficacy, came to offer a blueprint for expanding participatory channels at the UN to colonized peoples in the much larger list of Non-Self-Governing Territories (NSGTs).