Integration of mental health into routine primary health care (PHC) services in low-and middle-income countries is globally accepted to improve health outcomes of other conditions and narrow the mental health treatment gap. Yet implementation remains a challenge. The aim of this study was to identify implementation strategies that improve implementation outcomes of an evidence-based depression care collaborative implementation model integrated with routine PHC clinic services in South Africa. An iterative, quasi-experimental, observational implementation research design, incorporating the Reach, Effectiveness, Adoption, Implementation and Maintenance (RE-AIM) framework, was applied to evaluate implementation outcomes of a strengthened package of implementation strategies (stage two) compared with an initial evaluation of the model (stage one). The first stage package was implemented and evaluated in 10 PHC clinics and the second stage strengthened package in 19 PHC clinics (inclusive of the initial 10 clinics) in one resource-scarce district in the province of KwaZulu-Natal, South Africa. Diagnosed service users were more likely to be referred for counselling treatment in the second stage compared with stage one (OR 23.15, SE = 18.03, z = 4.04, 95%CI [5.03–106.49], p < .001). Training in and use of a validated, mandated mental health screening tool, including on-site educational outreach and technical support visits, was an important promoter of nurse-level diagnosis rates (OR 3.75, 95% CI [1.19, 11.80], p = 0.02). Nurses who perceived the integrated care model as acceptable were also more likely to successfully diagnose patients (OR 2.57, 95% CI [1.03–6.40], p = 0.043). Consistent availability of a clinic counsellor was associated with a greater probability of referral (OR 5.9, 95%CI [1.29–27.75], p = 0.022). Treatment uptake among referred service users remained a concern across both stages, with inconsistent co-located counselling services associated with poor uptake. The importance of implementation research for strengthening implementation strategies along the cascade of care for integrating depression care within routine PHC services is highlighted.