Despite an increasing focus on schools to deliver support and education around mental health and wellbeing, interventions are often not sustained beyond initial funding and research. In this review, the barriers and facilitators to sustaining mental health and wellbeing interventions in schools are explored. A systematic review was conducted using keywords based on the terms: ‘sustainability’, ‘school’, ‘intervention’, ‘mental health’, and ’emotional wellbeing’. Six online databases (PsycINFO, Embase, MEDLINE, British Education Index, ERIC, and Web of Science) and relevant websites were searched resulting in 6160 unique references. After screening, 10 articles were included in the review and extracted data were qualitatively synthesized using thematic analysis. Data synthesis led to the identification of four sustainability factors at the school level (school leadership, staff engagement, intervention characteristics, and resources) and one at the wider system level (external support). These factors were separated into 15 themes and discussed as barriers and facilitators to sustainability (for example, school culture and staff turnover). Most articles included no definition of sustainability, and nearly all barriers and facilitators were discussed at the school level. The findings suggest that more longitudinal and theory-driven research is required to develop a clearer picture of the sustainability process.