Online communities (OCs) as a new form of organizing are emerging with recent technologies such as digital platforms and social media. Although OCs are recognized as generative spaces for knowledge collaboration and innovation, we still know little about how OCs are created and sustained over time. To address this issue, this paper conducts a longitudinal field study of a healthcare online community hosted by a Canadian medical association. Building on the field observational data (before and after the launch of OC) and the digital trace data of the OC platform, this study explores how the OC evolves with the imbrication of design and emergence over the course of community development. This study contributes to novel understandings of OC development and provides practical implications on how to manage and sustain online communities.