Purpose This paper aims to use systems mapping as a tool to develop an organisation-wide approach to public mental health to inform strategic direction within a national public health agency. Design/methodology/approach Two workshops were facilitated with internal staff from a wide range of public health policy teams working in small groups to produce paper-based maps. These were collated and refined by the project team and digitised. Findings The approach engaged a range of teams in forming a shared understanding and producing a complex system map of the influences on population mental health and well-being, where current policy initiatives were addressing them and what the gaps and priorities were. Participants valued the approach which led to further study and organisational commitment to the whole system working as part of national public mental health strategy. Research limitations/implications The approach was limited to internal stakeholders and wider engagement with other sectors and community members would help further the application of complex system approaches to public mental health. Originality/value It was a valuable process for developing a whole-organisation approach and stimulating thinking and practice in complex system approaches. The paper provides a practical example of how to apply systems mapping and its benefits for organising public mental health practice.