BackgroundThe World Mental Health Surveys conducted by the World Health Organization (WHO) have shown that huge treatment gaps for severe mental disorders exist in both developed and developing countries. This gap is greatest in low and middle income countries (LMICs).Efforts to scale up mental health services in LMICs have to contend with the paucity of mental health professionals and health facilities providing specialist services for mental, neurological and substance use (MNS) disorders. A pragmatic solution is to improve access to care through the facilities that exist closest to the community, via a task-shifting strategy. This study describes a pilot implementation program to integrate mental health services into primary health care in Nigeria.MethodsThe program was implemented over 18 months in 8 selected local government areas (LGAs) in Osun state of Nigeria, using the WHO Mental Health Gap Action Programme Intervention Guide (mhGAP-IG), which had been contextualized for the local setting.A well supervised cascade training model was utilized, with Master Trainers providing training for the Facilitators, who in turn conducted several rounds of training for front-line primary health care workers. The first set of trainings by the Facilitators was supervised and mentored by the Master Trainers and refresher trainings were provided after 9 months.ResultsA total of 198 primary care workers, from 68 primary care clinics, drawn from 8 LGAs with a combined population of 966,714 were trained in the detection and management of four MNS conditions: moderate to severe major depression, psychosis, epilepsy, and alcohol use disorders, using the mhGAP-IG. Following training, there was a marked improvement in the knowledge and skills of the health workers and there was also a significant increase in the numbers of persons identified and treated for MNS disorders, and in the number of referrals. Even though substantial retention of gained knowledge was observed nine months after the initial training, some level of decay had occurred supporting the need for a refresher training.ConclusionIt is feasible to scale up mental health services in primary care settings in Nigeria, using the mhGAP-IG and a well-supervised cascade-training model. This format of training is pragmatic, cost-effective and holds promise, especially in settings where there are few specialists.Electronic supplementary materialThe online version of this article (doi:10.1186/s12913-015-0911-3) contains supplementary material, which is available to authorized users.