Project DAIRE was a randomised-controlled, factorial design trial which aimed to improve children’s health-related quality of life, wellbeing, food knowledge and dietary habits via two multi-component interventions: Nourish and Engage. Nourish was an intervention aiming to alter the school food environment, provide food-based experiences and expose pupils to locally produced foods. Engage was an age-appropriate cross-curricular food education intervention incorporating food, agriculture, nutrition science and related careers. The purpose of this study was to conduct a process evaluation to evaluate DAIRE implementation, mechanisms of impact (MOI) and context to elucidate trial results, and inform scalable implementation of the DAIRE approach for successful future rollout. Methods The Medical Research Council’s (MRC) framework for process evaluation was followed. Formal (questionnaires designed for process evaluation) and informal (researcher records and communications) methods were used to collect quantitative and qualitative data during the DAIRE trial in relation to process evaluation. Quantitative data were analysed using descriptive statistics and qualitative data via thematic analysis to identify key themes. Results Fifteen schools and 983 pupils ( n = 495 6–7 year olds/Year 3 and n = 488 10–11 year olds/Year 7) were recruited for the 6-month DAIRE intervention; a 100% retention rate was observed at the school level and the interventions had a high level of pupil and teacher acceptability. Nourish schools delivered a higher mean dose of intervention elements (61.4%) than Engage (50%) schools but, overall, mixed implementation of both interventions occurred. DAIRE produced change through four key MOI: social learning, experimental learning, interactive engaging content and real-life connections. Lack of time was the main contextual barrier to implementation and lack of financial cost to schools indicated as a potential facilitator. Conclusions This process evaluation helped to identify important findings related to implementation, MOI and context. The most effective elements of the interventions which should be maintained include provision of interactive and engaging intervention elements at no financial cost to the school. Findings also identified suggestions for improvement including provision of increased teacher training, support and planning time, content reduction to facilitate easy integration, and implementation across the full academic year. A sustainable funding and resourcing mechanism is required for successful future roll-out across the UK and beyond. Trial registrations The original trial referenced in this process evaluation is registered as follows: National Institute of Health (NIH) U.S. National Library of Medicine Clinical Trials.gov (ID: NCT04277312; retrospectively registered 11th February 2020).