In this study, I assume that sustainable entrepreneurs, i.e, entrepreneurs who tackle environmental and social problems while also creating economic wealth (Schaltegger and Wagner, 2011), act as institutional entrepreneurs, promoting divergent changes in the models, schemas and norms of a given organizational field. By combining both entrepreneurship research and institutional theory, the aim of this research is threefold: to uncover how sustainable entrepreneurs 1) identify, evaluate and decide to explore business opportunities; 2) mobilize resources through mechanisms known in the entrepreneurship literature as causation, effectuation, and bricolage, and 3) legitimate their businesses and change institutions so as their products/services get accepted. The method used in this research was the multiple case study to get rich and detailed data, making a thorough understanding of the historical process and context of the venture creation possible. Confirming prior research, the results suggest that entrepreneurs use both effectuation and causation logics to explore opportunities. However, the institutional theory lens in this study suggests that the use of causation logic is intended at gaining legitimacy and attracting stakeholders. On the other hand, results also suggest that theorization strategies used by sustainable entrepreneurs may take an effectual approach, changing its arguments in an experimental and iterative way as stakholders face new necessities and challenges. By combining entrepreneurship research to institutional theory, this study fills a literature gap in both areas.