The purpose of this paper is to explore the relationship between formal and non-formal processes of accountability in a public services context. The paper presents a case study of the impact of the Health and Social Care Act (2012) on the practices of Health and Wellbeing Board (HWB) members. It draws upon multiple data sources, including in-depth interviews with the members, comprehensive archival data published by the HWB (2011-2019), and observations of HWB public meetings. The concept of dynamic duality is introduced to offer a more holistic understanding of the relationship between formal and non-formal processes of accountability within a public services context. The case illustrates the role of formal and non-formal processes of accountability at a HWB in England. Moreover, the case study reveals the relationship and interaction between the formal and non-formal accountability processes and how they change and transform each other over time. We find that whilst non-formal accountability processes were strengthened by a historical legacy of partnership working, over time the dynamics at play led to the development of formal accountability processes through more sophisticated performance systems. The paper presents a new and more holistic conceptualisation, dynamic duality, on the potential relationship between formal and non-formal accountability processes that has applicability in the public services domain.