We offer a systematic account of the creation and design of transnational public-private governance initiatives (TGIs) in which states and/or intergovernmental organisations (IGOs) cooperate with business and civil society actors. Building on organisational ecology and regime complexity theories, we argue that states take into account the characteristics of the institutional context within which a new governance initiative will be situated when deciding whether to create a TGI and, if they decide to do so, how robust its institutional structure will be. States seek to minimize transaction costs and institutional redundancies and leverage the benefits of the complementarities of different institutional designs. As a consequence, TGIs are more likely when pre-existing IGOs perform similar functions in similar issue areas for similar sets of members and less likely when they overlap with similar pre-existing TGIs. If TGIs are created, similar prior IGOs incentivise states to choose low levels of organisational robustness to benefit from the flexibility of informal governance. We test these arguments using a new dataset on 636 TGIs and 534 IGOs created in the period 1815 to 2017 and statistical models that take into account selection effects. The results support our expectations and have implications for the study of transnational governance, regime complexes, and global governance.