This dissertation examines the effects of diversified resource mobilization on the agency of secretariats in International Organizations (IOs). While IOs are designed for longevity and resilience, they have seen dramatic shifts from assessed contributions to new actors and modalities. A theory of diversified resource mobilization in IOs addresses this tension between continuity and change. The theory operates at two levels of analysis. The group level captures effects of the number and complexity of funders. The dyadic level unpacks the effects of specific funders on the agenda and capacity of the secretariat. The empirical chapters of the dissertation apply this theory to the African Union (AU). A comparison of three budget categories (regular budget, peace and security, and infrastructure development) teases out variation in how diversified resource mobilization affects the agency of the AU’s secretarial organizations. The AU overall succeeded in increasing secretariat agency through diversified resource mobilization, even if this agency increase comes in parallel with conditionality by external funders. The dissertation informs debates on principal-agent analysis of IOs through a comprehensive theory of delegation through IO funding. As bureaucratic organizations, IO secretariats take a central role for delegated agency as well as organizational sources of agency. Understanding the AU’s funding, and that of countless other IOs with an institutionalized secretariat, requires looking beyond formal, state-based membership and assessed contributions, and positing IO secretariats as central actors in the transformation of resources into results.