Background The involvement of service users and caregivers is recommended as a strategy to strengthen health systems and scale up quality mental healthcare equitably, particularly in low-and-middle-income countries. Service user and caregiver involvement is complex, and its meaningful implementation seems to be a worldwide challenge. Theory of Change (ToC) has been recommended to guide the development, implementation and evaluation of such complex interventions. This paper aims to describe a ToC model for service user and caregiver involvement in a primary mental health care in rural Ethiopia. Methods The ToC was developed in two workshops conducted in (i) Addis Ababa with purposively selected psychiatrists (n = 4) and multidisciplinary researchers (n = 3), and (ii) a rural district in south-central Ethiopia (Sodo), with community stakeholders (n = 24). Information from the workshops (provisional ToC maps, minutes, audio recordings), and inputs from a previous qualitative study were triangulated to develop the detailed ToC map. This ToC map was further refined with written feedback and further consultative meetings with the research team (n = 6) and community stakeholders (n = 35). Results The experiential knowledge and professional expertise of ToC participants combined to produce a ToC map that incorporated key components (community, health organisation, service user and caregiver), necessary interventions, preconditions, assumptions and indicators towards the long-term outcomes. The participatory nature of ToC by itself raised awareness of the possibilities for servicer user and caregiver involvement, promoted co-working and stimulated immediate commitments to mobilise support for a grass roots service user organization. Conclusions The ToC workshops provided an opportunity to co-produce a ToC for service user and caregiver involvement in mental health system strengthening linked to the planned model for scale-up of mental health care in Ethiopia. The next steps will be to pilot a multi-faceted intervention based on the ToC and link locally generated evidence to published evidence and theories to refine the ToC for broader transferability to other mental health settings.