Although evidence-based treatments for depression in low-resource settings are established, implementation strategies to scale up these treatments remain poorly defined. We aimed to compare two implementation strategies in achieving high-quality integration of depression care into chronic medical care and improving mental health outcomes in patients with hypertension and diabetes. We conducted a parallel, cluster-randomised, controlled, implementation trial in ten health facilities across Malawi. Facilities were randomised (1:1) by covariate-constrained randomisation to either an internal champion alone (ie, basic strategy group) or an internal champion plus external supervision with audit and feedback (ie, enhanced strategy group). Champions integrated a three-element, evidence-based intervention into clinical care: universal depression screening; peer-delivered psychosocial counselling; and algorithm-guided, non-specialist antidepressant management. External supervision involved structured facility visits by Ministry officials and clinical experts to assess quality of care and provide supportive feedback approximately every 4 months. Eligible participants were adults (aged 18-65 years) seeking hypertension and diabetes care with signs of depression (Patient Health Questionnaire-9 score ≥5). Primary implementation outcomes were depression screening fidelity, treatment initiation fidelity, and follow-up treatment fidelity over the first 3 months of treatment, analysed by intention to treat. This trial is registered with ClinicalTrials.gov, NCT03711786, and is complete. Five (50%) facilities were randomised to the basic strategy and five (50%) to the enhanced strategy. Between Oct 1, 2019, and Nov 30, 2021, in the basic group, 587 patients were assessed for eligibility, of whom 301 were enrolled; in the enhanced group, 539 patients were assessed, of whom 288 were enrolled. All clinics integrated the evidence-based intervention and were included in the analyses. Of 60 774 screening-eligible visits, screening fidelity was moderate (58% in the enhanced group vs 53% in the basic group; probability difference 5% [95% CI -38% to 47%]; p=0·84) and treatment initiation fidelity was high (99% vs 98%; 0% [-3% to 3%]; p=0·89) in both groups. However, treatment follow-up fidelity was substantially higher in the enhanced group than in the basic group (82% vs 20%; 62% [36% to 89%]; p=0·0020). Depression remission was higher in the enhanced group than in the basic group (55% vs 36%; 19% [3% to 34%]; p=0·045). Serious adverse events were nine deaths (five in the basic group and four in the enhanced group) and 26 hospitalisations (20 in the basic group and six in the enhanced group); none were treatment-related. The enhanced implementation strategy led to an increase in fidelity in providers' follow-up treatment actions and in rates of depression remission, consistent with the literature that follow-up decisions are crucial to improving depression outcomes in integrated care models. These findings suggest that external supervision combined with an internal champion could offer an important advance in integrating depression treatment into general medical care in low-resource settings. The National Institute of Mental Health.