In response to increasing concerns about the mental health and wellbeing of university students and staff, policy aims have shifted towards a 'whole-university approach' to mental health and wellbeing. This policy advocates for a culture wherein mental health and wellbeing are a key priority across all levels (individual, societal, environmental). Thereby, responsibility for mental health and wellbeing is distributed across the institution, requiring contributions from staff irrespective of whether mental health and wellbeing are central to their role. Consequently, boundaries of responsibility and expertise can be unclear, while individual and professional capacity and opportunity influence the consistency of support. Effective governance of the whole-university approach requires an understanding of the complex network of 'actors' in the wellbeing system to cohesively deliver strategic objectives. This mixed-methods case study of one Higher Education Institution (HEI) employed social network analysis (SNA) to identify network structures and connections between staff who promote mental health and wellbeing. Qualitative follow-up explored factors associated with network prominence, cohesion between the informal network and formal structures, and overall perceptions of the network. An informal network of 211 actors in the wellbeing system was identified, revealing disparities with formal governance structures. Prominence in the network was attributed to both extrinsic (e.g. workplace culture and leadership) and intrinsic (e.g. social rewards) factors, and was perceived to provide value by increasing cohesive and collaborative working. However, findings also indicate the need to raise awareness of the network and improve capacity for network membership and engagement.