Scholars of international relations have long studied the rapid proliferation of new institutions across areas of global governance, and whether institutional proliferation leads to fragmentation and the undermining of legacy state-led regimes. As a response, new concepts and frameworks have been introduced to better understand changing global governance arrangements including regime complexes, governance architecture, and the application of organizational ecology to international affairs. However, we argue existing frameworks have critical limitations for analyzing global governance and characterizing the degree of fragmentation over time, often failing to simultaneously address sequencing, interactions between institutions, and the diversity of actors involved. We propose a methodological alternative to measuring fragmentation that maps which actors take on which governance functions when, across evolving sub-areas of a governance complex. Using the case of sustainable finance governance, we show the utility of the framework in identifying institutional competition and diversification. In the case of sustainable finance, results suggest that institutional proliferation is more often a matter of diversification than functional fragmentation or competition, and proliferation is driven as much by international organizations as entrepreneurial non-state actors. Though the substantive results may re-frame our understanding of sustainable finance at the global scale, our approach also offers a useful methodological starting point for comparative analysis of global governance complexes, an important frontier in international relations research.