Digital peer-to-peer service platform organizations need to mobilize users’ trust in order to attract and retain users. While prior research has provided a rich picture of the use and effec-tiveness of platform-inherent institutional mechanisms to build trust, we know little about how institution-based trust is mobilized in the early stages of platform organizations, when those mechanisms are not yet effective. Drawing on in-depth qualitative data from early-stage peer-to-peer service platform organizations in Panama and Mexico, we find that platform entrepre-neurs engage in a delicate process of “grounding institution-based trust”—iteratively combining technology- and data-based mechanisms with practices that eliminate high risks, personally familiarize users, and leverage platform communities. The process model we develop shows how these practices depend on one another, how they develop over time and gradually enable institution-based mechanisms to become effective. We contribute to our understanding of trust in the context of digital peer-to-peer platform organizations, of institution-based trust and its production more generally, and of resource mobilization in early-stage organizations.