Background: The integration of mental health services into primary health care (PHC) is considered a key strategy to improve access to care for people with common mental disorders (CMDs) in low- and middle-income countries (LMICs), yet mental health services remain largely unavailable at the PHC level. In Mexico, mental health services are only available at 30% of PHC clinics. Difficulties in translating research findings into routine health service delivery represent a major obstacle to integration of mental health care in PHC. This project investigated the barriers and facilitators to the implementation of mental health programmes integrated in PHC platforms in low-resource settings. Methods: I conducted a systematic review of the barriers and facilitators to the implementation of programmes for CMDs in primary care in LMICs. Then I conducted a mixed-methods case study of a mental health programme integrated in PHC clinics located in rural Mexico to examine implementation process and outcomes, and elicit potential barriers and facilitators to the programme implementation. First, I used mixed-methods to describe the programme implementation and examine outcomes. Subsequently, I used mixed-methods to explore factors related to non-attendance to mental health follow-up consultations. Finally, I used qualitative methods to elicit barriers and facilitators to implementation from the perspectives of service providers and service users. Results: Factors influencing programme implementation were identified through the systematic review including the organisation’s readiness for implementation, the attributes, knowledge and beliefs of service providers, complex service user needs, adaptability and perceived advantage of interventions, and the processes of planning and evaluating the implementation. The case study showed that implementation outcomes included: programme integration, and high levels of acceptability and feasibility enabled through support from the implementing organisation. Fidelity was limited due to the low provision of talk-based interventions. Providers identified that delivering talk-based interventions was unfeasible due to time constraints and limited specialist support to develop the skills needed to provide them. Non-attendance to mental health consultations was an important challenge to implementation fidelity. Main barriers to attendance included, long distance to the clinics, type of treatment, and waiting times; facilitators were the presence of a comorbidity, and perceived need of treatment. Experiences with providers or treatments were identified as both facilitators and barriers. Key facilitators to the programme implementation were the cultural adaptation and perceived advantage of interventions to deliver mental health care, the commitment of health providers, the availability of key resources, an organisational culture that promoted health care as a human right, and the presence of a strong programme leadership. Key barriers included the complexity of mental health interventions, low self-efficacy from health providers, insufficient availability of mentorship from specialists, and the complex needs and expectations of service users. Conclusions: Strengthening the health system is a necessary first step to implement mental health programmes in PHC to ensure ongoing capacity building mechanisms, essential resources, and specialist support are available. Moreover, to adequately address the health and social needs of service users in low-resource settings, locally relevant social interventions and intersectoral collaboration are essential.