Recent international relations scholarship has adopted the perspective of organizational ecology (OE) to explore a range of questions related to organizational emergence, strategy, and death. These studies draw attention to organizational competition as the mechanism underpinning important transformations in global governance. We argue that existing work in IR that uses OE has overlooked the importance of another strand of sociological theory that focuses on dynamics of mutualism between organizations. We illustrate the importance of mutualism by focusing on a crucial case: the evolution of different “populations” of organizations working in environmental governance during its critical 1970–1990 period. Our analysis demonstrates that as the environmental consciousness of the 1970s took hold, international non-governmental organizations (INGOs) increasingly captured new resources and stimulated new attention to the issue. Rather than viewing these new actors as competition, existing international organizations (IOs) sought to incorporate and legitimate INGOs, promoting their growth. And in turn, INGOs sought to support and legitimate the activities of the existing IOs, promoting growth of Secretariats and treaties. Our account offers an important organizational-level story that shows that dynamics of mutualism help account for the increased complexity of global governance.