Aims/hypothesis <p>Ethnic disparities in the incidence of type 2 diabetes mellitus are well documented in multiethnic urban populations, but the contributions of migration status and mental health are less well understood. This study used a large dataset from primary care centres in South London that is unique in that it includes migration-related information together with information on mental and physical health comorbidities. We aimed to assess how migration status and mental health contribute to longitudinal associations of ethnicity and type 2 diabetes risk in a multiethnic urban population.</p> Methods <p>We conducted a longitudinal cohort study (2012–2019) of approximately 340,000 adults without baseline type 2 diabetes. Cox proportional hazards models were applied with sequential adjustments: first for age and sex; second, adding migration status (country of birth being UK or not); and third, further adding mental health conditions (depression, anxiety, severe mental illness), physical health factors (BMI, hypertension and other macrovascular diseases) and area-level deprivation. This approach allowed us to examine whether ethnic differences in the incidence of type 2 diabetes persist after accounting for additional factors.</p> Results <p>South Asian, Black African and Black Caribbean groups had 2–3-fold higher type 2 diabetes risks compared with White British individuals, which were only partially explained by socioeconomic and clinical factors. Being born outside the UK increased type 2 diabetes risk by 29% across all ethnic groups. Depression/anxiety and severe mental illness were associated with a higher risk of type 2 diabetes. No statistical evidence of strong interactions between these factors was obtained.</p> Conclusions/interpretation <p>Ethnicity, migration status and mental health conditions were each independently associated with type 2 diabetes risk, and ethnic disparities persisted after adjustment. The lack of evidence for interactions suggests that migration- and mental health-related mechanisms may operate similarly across ethnic groups rather than amplifying or mitigating existing disparities in type 2 diabetes rates. Efforts to reduce diabetes inequalities will require both support for post‑migration challenges and addressing of the broader structural and environmental determinants underlying persistent ethnic disparities.</p> Graphical Abstract <p></p>