Several billionaires have recently emerged as leaders of climate governance. So far, little research has examined how they legitimize their involvement in climate networks. We argue that billionaire governance entrepreneurs have high levels of resources but low procedural legitimacy. They pursue output legitimacy to support their political action, highlighting their effectiveness in managing climate issues. Their main strategies, depoliticization, outgrouping, and technical solutionism, may give them short-term legitimacy but risk undermining their long-term goals of addressing climate change. We analyze the discursive legitimation strategies of a successful billionaire entrepreneur in transnational climate governance, Michael Bloomberg. Our empirical analysis is based on the study of more than 800 statements, speeches, and news releases related to Bloomberg’s climate action from 2010 to 2021. It contributes to the study of entrepreneurship, leadership, philanthropy, and transnational actors in climate governance.