How do international organizations (IOs) govern transnational challenges? Most theories maintain that IOs exercise authority to govern. What these authority-focused accounts tend to overlook, however, are instances of de facto governance. Especially in emerging, contested, and crisis-ridden issue areas, authority has often not been established or become unsettled. Yet, IOs govern here, too. Take the example of migration and asylum: This policy field is characterized by institutional and policy gaps. During the crisis at Europe’s border in 2015–2016, IOs governed mixed movements nonetheless. Through organizing collective action on the ground, they not only created direct regulative impacts on the lives of people on the move (the final addressees of international politics) but also defined what mixed migration means as a global policy concern. I draw on practice theory and fieldwork at the European external border in Greece to draw attention to governing modes that operate at a very low institutional threshold. I propose a minimal conception of governance that shifts attention from authority sources to governing effects to account for such governance forms. This re-conceptualization makes the study of how IOs govern outside their established authority, in concrete geographical places, possible.