Background and Objectives: In Australia, the Royal Commission into Victoria’s Mental Health System identified a complex and fragmented system ill-equipped to address the needs of people living with mental illness or psychological distress, along with their families, carers and supporters (1, 2). To begin to address these systemic failings, recommendations included calls for co-designed systems-level innovations responsive to the voice of those with lived-experience (3, 4). Group model building (GMB) is a participatory systems-based approach to enhance problem understanding, increase engagement and co-design systemic solutions. This research aimed to assess the usefulness and acceptability of GMB as a process for co-design within the mental health sector and provide clear translation of the methodologies. Methods: A scoping review on participatory methods used within mental health services informed the co-design approach. Focus groups and GMB workshops with health service lived-experience workforce, clinicians, and other healthcare workers were run to identify person-centred growth-oriented language to be used, acceptability of existing GMB Methods, and the co-creation of adaptions to improve efficacy as a methodological approach. Results: Person-centred growth-oriented language identified and adopted into vocabulary. A co-created systems approach for stakeholder engagement tailored to the context of mental health services. Conclusions: This research seeks to elevate people with lived experience to transform mental health services from within, delivering an evidence based, person-centred and co-designed approach to mental health service improvement. This co-created systems approach for stakeholder engagement will be piloted with an acute mental health inpatient unit to ascertain and foster a shared understanding of the problem scenario and identify contextually relevant and sustainable intervention strategies.