Effective school-based mental health promotion and prevention interventions in low-and middle-income countries (LMICs) can positively impact the mental health and well-being of large numbers of young people. This scoping review aimed to investigate the implementation of effective mental health promotion and prevention interventions in LMIC schools. A scoping review of the international literature was conducted and followed the Preferred Reporting Items for Systematic reviews and Meta-Analysis extension for Scoping Reviews guidelines. Medline, PsycInfo, Scopus, Embase, CINAHL and Cochrane were searched for peer-reviewed literature published from 2014 to 2022. PsycExtra, Google Scholar and the websites of key organisations were searched for relevant grey literature. Study selection focussed on mental health promotion interventions, including the development of social and emotional skills and mental health literacy, and prevention interventions, including anti-bullying and skill-based interventions for "at-risk" students. Twenty-seven studies evaluating 25 school-based interventions in 17 LMICs were included in the review. Fifteen interventions were developed in the implementing country and 10 were adapted from high-income countries (HICs) or other settings. Findings from the studies reviewed were generally positive, especially when interventions were implemented to a high quality. Universal life-skills interventions were found to increase social and emotional skills, decrease problem behaviours and positively impact students' mental health and well-being. Mental health literacy interventions increased mental health knowledge and decreased stigma among students and school staff. Outcomes for externally facilitated anti-bullying interventions were less positive. All 19 effective studies reported on some aspects of programme implementation, and 15 monitored implementation fidelity. Eleven studies outlined the programme's underpinning theoretical model. Only four studies reported on the cultural adaptation of programmes in detail. Including young people in the adaptation process was reported to facilitate natural cultural adaptation of programmes, while input from programme developers was considered key to ensuring that the core components of interventions were retained. The review findings indicate increasing evidence of effective mental health interventions in LMIC schools. To facilitate the sustainability, replication and scaling-up of these interventions, greater attention is needed to reporting on intervention core components, and the processes of implementation and cultural adaptation in the local setting.