BACKGROUND: Group model building (GMB) is a participatory approach whereby diverse stakeholders can share their views about a problem to create a collective understanding of a complex system. In this article we report our methodological approach to adapt face-to-face GMB processes to an online format to explore the mechanisms by which commercial drivers influence adolescents' dietary behaviour. We use our experiences to make recommendations on how online GMB could be delivered. METHODS: We planned, adapted, piloted, delivered, and evaluated a series of online GMB workshops with adolescents, policymakers and public health practitioners to create a system map of the commercial determinants of dietary behaviour in adolescence. We adapted face-to-face GMB workshops to a series of 3 online GMB workshops with 11 adolescents (16-18 years) living in the Southwest of England, and one GMB workshop with policymakers and public health practitioners. RESULTS: In our experience, adapting, and delivering GMB online is feasible, engaging, cost-saving and an enjoyable experience. Participants gave positive feedback in terms of engagement and enjoyment, and it allowed them to recognise different points of view about the same problem. Participants became familiarised with system thinking and system dynamics concepts, developed a shared understanding of a complex issue and portrayed it in a system map that depicted the most important factors in a causal structure as well as their interactions at various levels. CONCLUSIONS: We recommend using process mapping to understand the overall GMB process in an online environment and piloting the workshops to test the timings and flow between online platforms. For facilitation and delivery, facilitators need to ensure they can create an inviting and engaging online environment, even for participants who decide to have their cameras off. Separating GMB activities into different workshops allowed participants to reflect on the problem being discussed and bring new ideas to subsequent workshops. Evaluating the workshops enabled us to build evidence on the trade-offs between the effectiveness, quality and efficiency of online GMB workshops, and how this might be enhanced to identify leverage points and achieve systemic changes in complex issues. ETHICAL APPROVAL: The research was approved by the University of Bristol's Faculty of Health Sciences Research Ethics Committee (Ref: 99,003) and written consent was received from all participants.