Background: The impact of occupational therapy on mental health outcomes for children is largely unexplored. The aim of this study was to investigate an evidence-based occupational therapy intervention designed to increase participation in daily occupations in order to prevent symptoms of mental illness for children and run in schools. Methods: The study used a pragmatic, cluster-randomised controlled trial design with two arms. Fourteen clusters (schools), equating to 151 child participants, were stratified by school decile-rank category and block randomised. Blinding of participants post-randomisation was not feasible; however, outcomes assessors were blinded. Outcomes were measured at baseline, after the parallel and crossover phases, and at follow-up; and were anxiety symptoms (primary), depression symptoms, self-esteem, participation and wellbeing. Intention-to-Treat analysis was applied -mixed linear modelling was used to account for clusters and repeated measures, and to adjust for covariates identified. Results: This trial found significant positive effects of the intervention on child-rated satisfaction with their occupational performance and teacher-rated child anxiety. No evidence was found to support the effect of the intervention on anxiety and depression symptoms, self-esteem and wellbeing. Conclusions: This was the first known cluster-randomised controlled trial to investigate an occupational therapy intervention promoting emotional wellbeing in a non-clinical sample of children. No compelling evidence was found to support the use of the intervention in schools in its current format, however, results were promising that the focus on occupations influenced participation. Recommendations are made to redesign the intervention as an embedded intervention in the classroom, co-taught by teachers and including parental involvement.