There is limited research on predictors of poor mental health for migrant women. This mixed-methods study investigated: 1) the impact of migration and stressful life events on the risk of common mental disorders and Post Traumatic Stress Disorder; 2) what experiences women perceive have impacted on their mental health and well-being, and how they have been affected. Methods: Cross-sectional survey (South East London Community Health Study) using a standardised diagnostic clinical interview (CIS-R) and other validated measures; qualitative interviews with a purposive sample of immigrant women and women born in the UK. Results: 391 immigrant women and 553 women born in the UK were included in the survey. There was a high prevalence of potentially traumatic events among migrant women (66.5%; 95% CI 61.4 71.3) and women born in the UK (71.4%; 95% CI 67.2 75.2), which increased the risk of poor mental health. However, there was no significant difference in the odds of poor mental health for migrant women compared with women born in the UK (AOR 1.0; 95% CI 0.7-1.6). 20 immigrant women and ten women born in the UK participated in the qualitative interviews. Abuse and powerlessness were found to negatively impact on women's mental health and well-being; legal status, financial instability, and isolation were particularly salient barriers to coping for migrant women. Conclusions: Stressful life events (e.g. abuse) and psychosocial barriers (e.g. powerlessness) impact on women's mental health; migrant women may experience specific barriers to help-seeking. These factors need identification by healthcare professionals.