Mental health is an immediate concern for public health and government officials due to the increase in mental ill health issues among young people and adults. School-based mental health services (SBMHSs) have grown rapidly in recent years, in response to this, as policy debates focus on the health and economic implications of mental for individuals and society. School-based mental health programs are focused on identifying, intervening, and providing mental health support to children and young people through the promotion of mental health and well-being within schools. With their increased presence and funding in both high- and low-income countries, concerns have been raised regarding their effectiveness, implementation, and sustainability as a means of early intervention and reducing adult mental health issues. While there are some promising results on the effectiveness of SBMHS, many barriers have been identified which hinder the implementation and sustainability of these services. Barriers include cultural and contextual factors for local adaptation; the specifics of intervention delivery; school capacity, leadership, and commitment; and system-level factors such as frameworks, policies, and government support. The research findings highlight that barriers and facilitators for the implementation and sustainability of school-based mental health interventions are not dissimilar to those observed in health, improvement, and educational interventions. As such, further research is needed to provide clarity on the principles needed to implement and sustain school-based mental services which effectively provide relevant and timely access to mental health support and services to children and young people.