ObjectiveTo measure and understand mental wellbeing among women prisoners in Chile, as part of a larger study.ResultSixty-eight sentenced prisoners in a women’s prison participated in a survey, giving a response rate of 56.7%. Using the Warwick-Edinburgh Mental Wellbeing Scale (WEMWBS), the mean wellbeing score of participants was 53.77 out of maximum score of 70. Whilst 90% of the 68 women felt useful at least some of the time, 25% rarely felt relaxed, close to others or able to make up their own minds about things. Data generated from two focus groups attended by six women offered explanations for survey findings. Thematic analysis identified stress and loss of autonomy due to the prison regime as factors which negatively affect mental wellbeing. Interestingly, whilst offering prisoners an opportunity to feel useful, work was identified as a source of stress. Interpersonal factors linked to a lack of safe friendships within the prison and little contact with family had an adverse impact on mental wellbeing. The routine measurement of mental wellbeing among prisoners using the WEMWBS is recommended in Chile and other Latin American countries to identify the impact of policies, regimes, healthcare systems and programmes on mental health and wellbeing.