Introduction: Integration of mental health services into Primary Health Care (PHC) is a proven way of reducing the treatment gap in developing countries. A major constraint to scaling up mental health services in developing countries is scarcity of mental health professionals. A practical solution is to adopt task shifting and task sharing strategies involving Primary Health Care Workers (PHW). One of the major challenges of such integrative services is their long term outcomes and sustainability. The Neuropsychiatric Hospital Aro, Abeokuta, Nigeria embarked on mental health services provision across primary health care facilities in Ogun state six (6) years ago. Objective: This report describes the development, challenges of the programme and presents a post-implementation evaluation after 6 years of its commencement. Methods: Applying a population based expansion of pilot- tested integration model of Aro Primary Care Mental Health Programme (APCMHP) for Ogun State, 80 PHC workers were trained using an adapted mental health Gap Action Programme (mhGAP) intervention guide to assess and treat/refer 5 priority conditions: Psychosis, Depression, Epilepsy, Alcohol and Substance abuse and Other Significant Emotional Complaints (OSEC).There was mental health service provision in 40 designated PHC centers across Ogun state. There was support and supervision of the trained health workers by field supervisors, supplementary training and re-training for skill sustenance, periodic stakeholders meeting with Local Government Service Commission, zonal consultants’ review, financial and other resources commitment by the hospital, monthly programme evaluation and monitoring by the faculty members. We reviewed caseload of patients managed by trained PHC Workers since commencement of the programme in November 2011 till October 2017 (6 years period) using descriptive statistics. Appropriate ethical approval was obtained. Results: During the six-year period (November 2011-October 2017), 2194 cases (average of 366 new cases yearly) were identified and treated by Trained Health Workers (THWs). About 90% of cases were Psychosis and Epilepsy. There was a steady attrition of THWs and at the end of the sixth year only 29% of the THWs remained within the programme. Treatment outcomes were fair as over 50% of patients had ≥ 3 follow-up visits, symptom remission of ≥ 30% and a subjective improvement in Global Ratings. Conclusion: Our project has demonstrated that it is feasible, practicable and cost effective with community acceptance to scale up mental health services at primary care setting in Nigeria using adapted mhGAP-IG document. The need to understand the dynamics and econometrics of sustainable primary mental health services is indicated.