Background: In the UK, there has been a strong drive towards patient and public involvement (PPI) in health research. Its benefits include improvements in the quality, relevance and acceptability of research, and empowerment, self-respect and value for service users. Organisational context can significantly influence the operationalisation of PPI. Research has highlighted power asymmetries between clinicians, researchers and service users. A resistance to power sharing, tokenism and assimilation into the existing culture suggest that a consultative, technocratic form of PPI is operating within health research settings. The aim of the study was to explore the development of PPI within a London based mental health biomedical research centre (BRC) over a period of 10 years from its inception. Methods: This qualitative study compared data from 52 organisational documents and 16 semi-structured interviews with staff and service users associated with PPI within the Maudsley BRC. The data were analysed using inductive thematic analysis. Study design, data collection, analysis and write up were conducted by service user researchers. Results: Our analysis showed a picture of increasing activity and acceptance of PPI, its alignment with the broader BRC research agenda, progressive involvement of service users in governance, and the development of a collaborative culture in research processes. The presence of salaried service user researchers in the organisation was key to this progress. However, PPI remained localised and under resourced and there was a reluctance to change working practices which resulted in perceptions of tokenism. Service users faced conflicting expectations and were expected to assimilate rather than challenge the organisation's 'biomedical agenda'.