BACKGROUND: Scaling up mental healthcare through integration into primary care remains the main strategy to address the extensive unmet mental health need in low-income countries. For integrated care to achieve its goal, a clear understanding of the organisational processes that can promote and hinder the integration and delivery of mental health care is essential. Theory of Change (ToC), a method employed in the planning, implementation and evaluation of complex community initiatives, is an innovative approach that has the potential to assist in the development of a comprehensive mental health care plan (MHCP), which can inform the delivery of integrated care. We used the ToC approach to develop a MHCP in a rural district in Ethiopia. The work was part of a cross-country study, the Programme for Improving Mental Health Care (PRIME) which focuses on developing evidence on the integration of mental health in to primary care. METHODS: An iterative ToC development process was undertaken involving multiple workshops with stakeholders from diverse backgrounds that included representatives from the community, faith and traditional healers, community associations, non-governmental organisations, Zonal, Regional and Federal level government offices, higher education institutions, social work and mental health specialists (psychiatrists and psychiatric nurses). The objective of this study is to report the process of implementing the ToC approach in developing mental health care plan. RESULTS: A total of 46 persons participated in four ToC workshops. Four critical path dimensions were identified: community, health facility, administrative and higher level care organisation. The ToC participants were actively engaged in the process and the ToC encouraged strong commitment among participants. Key opportunities and barriers to implementation and how to overcome these were suggested. During the workshops, a map incorporating the key agreed outcomes and outcome indicators was developed and finalized later. CONCLUSIONS: The ToC approach was found to be an important component in the development of the MHCP and to encourage broad political support for the integration of mental health services into primary care. The method may have broader applicability in planning complex health interventions in low resource settings.