This chapter focuses on the United Nation’s largest development entity, the United Nations Development Programme (UNDP), and asks: When and why has it integrated climate adaptation into its mandate? It traces UNDP’s evolving adaptation mandate from 1990 to 2015, drawing on over fifty interviews and an extensive analysis of primary documents. It argues that UNDP Administrators, rather than states, played a critical role in mandate expansion. Administrators decided whether and how to integrate adaptation into UNDP’s mandate and subsequently lobbied states to endorse any expansion. It also suggests that UNDP’s expansion was facilitated by its early access to multilateral climate trust funds. This chapter makes an important contribution to existing theories of international bureaucracies, which often assume that organizational change is state-driven (statist explanations) or that bureaucracies will always seek to expand (principal–agent and constructivism). Overall, it suggests scholars should look at how leaders navigate financial, ideational, and normative environment to understand change and influence in international institutions.