Digital platforms offer a promising contemporary means for encouraging social innovation through cross-sector collaboration. Yet although such “social-mission platforms” are equipped to facilitate a high number of arms-length transactions, they are conversely ill-equipped to provide the necessary consensus which typically characterizes successful examples of cross-sector collaboration. Employing an in-depth archival case study of a civic crowdfunding platform, we surface a process model of social-mission platform creation, which exposes the dilemmas such platforms encounter as they attempt to navigate user growth, and the importance of institutional infrastructure for overcoming these dilemmas. These findings and our emergent model thus contribute new theory regarding the creation of digital platforms for enabling cross-sector collaboration and social innovation, while bridging the emerging body of research on platforms with institutional theory