Objective Living Library 'Readers' can learn about experiences of others through conversations with living 'Books'. This study sought to generate a realist programme theory and a theory-informed implementation guide for a Library of Lived Experience for Mental Health (LoLEM). Design Integrated realist synthesis and experience-based co-design. Setting Ten online workshops with participants based in the North of England. Participants Thirty-one participants with a combination of personal experience of using mental health services, caring for someone with mental health difficulties, and/or working in mental health support roles. Results Database searches identified 30 published and grey literature evidence sources which were integrated with data from 10 online co-design workshops. The analysis generated a programme theory comprising five context-mechanism-outcome (CMO) configurations. For Readers, direct conversations humanise others' experiences (CMO 2) and provide the opportunity to flexibly explore new ways of living (CMO 3). Through participation in a Living Library, Books may experience personal empowerment (CMO 4), while the process of self-authoring and co-editing their story (CMO 5) can contribute to personal development. This programme theory informed the co-design of an implementation guide highlighting the importance of tailoring event design and participant support to the contexts in which LoLEM events are held. Conclusions The LoLEM has appeal across stakeholder groups and can be applied flexibly in a range of mental health-related settings. Implementation and evaluation are required to better understand the positive and negative impacts on Books and Readers. Registration PROSPERO CRD42022312789 Strengths and limitations of this study - This study used a novel, iterative, and creative approach to integrating theory development and intervention co-design. - The programme theory provides a conceptual basis for further evaluation of a LoLEM, including outcomes for those participating in events as 'Books' and 'Readers'. - This study informed detailed, co-designed implementation guidance for use by LoLEM organisers. - However, the LoLEM is yet to be delivered as a sustained programme of events, so long-term impacts require further investigation. Keywords Mental Health Services, Psychosocial Intervention, Qualitative Research, Social Support
