Digital Mental Health and Peer support has the potential to bridge gaps in support through its scalability and accessibility. Despite the increasing use of these platforms, there remains a lack of understanding of how they operate in real life, from initial engagement to longer-term impact. We aimed to explore the key inputs, processes, user interactions, assumptions, barriers, facilitators, outcomes, and impacts associated with the use of DMH and peer support platforms by developing a Theory of Change with stakeholders. Stakeholders (n = 77) contributed to the formulation of the Theory of Change through a series of online workshops, focus groups, interviews, and open-ended survey feedback. Workshops were structured to capture information related to aspects of the Theory of Change and to allow stakeholders to provide feedback to improve the diagram. A thematic framework approach was used to analyze transcripts to enable comparisons of factors reported by members, commissioners, and platform staff. Stakeholders identified a variety of factors contributing to initial inputs, processes, outcomes, and impact. Engagement emerged as the most significant barrier to the use of platforms. Motivations for use included filling in gaps in available support, connecting with others and upskilling. Different member types determined how users would interact with the platform which could influence the social response of others. Outcomes were largely positive including provision of a safe online space, improvement in wellbeing, and feeling connected to others. Stakeholders noted impact was harder to identify due to the preventative nature of these platforms but suggested this related to the knowledge of available support, reduction in waiting for support and in referrals, and increasing engagement and uptake of the platforms. Stakeholders identified assumptions regarding internet access as a significant barrier. The Theory of Change illustrated three distinct pathways in digital mental health and peer support. Further research is needed to improve engagement and factors influencing engagement, the member experience and how impact is measured.