Background: Low engagement with mental health treatment, including a lack of help-seeking and low subsequent attendance to treatment, are important challenges to the provision of mental healthcare. In settings with high levels of poverty, such as rural Mexican communities, low engagement may be aggravated by the limited availability of mental health services and existing barriers to access these services. This qualitative study was conducted to investigate factors that influenced engagement to a primary care mental health service programme in Chiapas, Mexico. Methods: We conducted 30 semi-structured interviews with adult service users diagnosed with depression or anxiety who had received mental health services at 10 primary care clinics in rural communities in Mexico. Interviews were analysed through the framework analysis approach to understand service user perspectives on factors influencing: (a) help-seeking, and (b) subsequent treatment attendance. Results: Lack of attribution of symptoms to a mental illness, the perception that symptoms could be managed without treatment, and stigma hindered help-seeking. Perceived need for care, prompted by strong feelings of desperation and severe somatic symptoms, along with advice from trusted sources about the availability of relevant services at the primary health care clinics, were key incentives for service users to make their first contact with the clinics. An ongoing desire for symptom alleviation and satisfaction with services (e.g. due to the positive impact of treatment) improved subsequent attendance, while feeling that treatment was no longer necessary and structural barriers led to non-attendance. Conclusions: Strategies to enhance understandings between service users and health care providers are necessary to improve the extent to which the needs and preferences of service users are met by healthcare services. Further improvements in the mental health of communities are likely to require interventions outside the health setting that tackle social risk factors affecting mental health.