BACKGROUND Improving mental health is a globally unmet medical need with close to 1 billion people living with a mental disorder. The UK national health system (NHS) introduced the Improving Access to Psychological Therapies (IAPT) program in 2007, which provides nationwide, community-based intervention to tackle common mental disorders (CMDs). All IAPT services systematically collect longitudinal patient data at every appointment resulting in the biggest standardised data resource on mental health care provision in the world. AIMS Address the key determinants of outcome in IAPT and establish the associations between ethnicity, social function and psychological treatment outcomes. Test the potential of the WSAS as an additional outcome metric. METHODS A systematic literature review was conducted to identify predictors of psychotherapy outcomes for CMDs. These were tested in a large retrospective IAPT cohort (N=78,986) by using multivariate logistic regression models. The statistical associations of the predictor variables, and specifically in ethnic subgroups, with Recovery and Reliable Improvement as the main outcomes were reported. The work and social adjustment scale (WSAS) score was tested as a novel outcome metric using a real change index (RCI) approach for improvement to calculate Reliable WSAS Improvement. FINDINGS Black and Minority Ethnic (BME) status is negatively associated with treatment outcomes. Adjusting the models for confounding factors such as initial morbidity, social function and occupation results in a weaker association for most ethnic groups. Some patients achieved improved social function despite not reaching treatment outcome targets for anxiety and depression. CONCLUSIONS Negative confounders for treatment outcomes are disproportionately reported in BME patients, suggesting multiple disadvantages associated with ethnicity. Improving social function is in itself an important aim of treatment and the Reliable WSAS Improvement is a derived variable that could work well as an additional outcome measure.