Experience-based co-design (EBCD) is a service design strategy that facilitates collaborative work between professional staff and service users toward common goals. There is a lack of published examples of it in relation to family carer engagement within a mental health context, and little research exploring the mechanisms behind successful implementation. The aim of this study was to explore the processes that facilitated EBCD with carer involvement. The study adopted a grounded theory–informed approach involving interviews with 16 participants of an existing EBCD project in an English National Health Service (NHS) trust, reflecting multiple stakeholders. EBCD can be thrown off track in two ways: conflict and getting “bogged down.” Leadership by project and design-group leaders could return group cohesion and maintain project momentum. The developed model reflects key processes. Future research should examine EBCD projects with similar ranges of stakeholders and in contexts with different levels of organizational change.