Autistic children commonly experience mealtime difficulties; however, it is unknown what makes an effective transdisciplinary mealtime service for autistic children. This study used a realist evaluation framework to explore caregivers’ and therapists’ perceptions of the contexts, mechanisms and associated outcomes of a transdisciplinary mealtime management service for autistic children. Semi-structured interviews were conducted with six caregivers of autistic children engaged in a transdisciplinary mealtime service and 10 therapists providing the service, with interview data analysed using a realist evaluation approach. Four-context (child factors, family factors, therapist factors and institutional parameters), three-mechanism (transdisciplinary collaboration, child-centred approach and managing caregiver expectations) and two-outcome (impact on the child and impact on the caregiver) themes were identified. Findings highlighted that transdisciplinary collaboration, a child- and family-centred approach and managing caregiver expectations are key mechanisms providing a foundation for the development of evidence-based mealtime services for autistic children. Lay abstract Mealtimes and eating can be difficult for autistic children. A service where different professions work together is required to address the varied and complex mealtime difficulties of autistic children. Little is known about what is needed for such services to be effective. We interviewed six caregivers of autistic children who were engaged in a mealtime service and 10 therapists who are involved in delivering the service to understand their perspectives on the factors that were driving the effectiveness of the mealtime service. We found that different health professionals from different disciplines working together, focusing on adapting intervention to the child and family and managing expectations of the caregiver were important in contributing to outcomes of the mealtime service. The findings of this study can be used to inform the development of more effective interventions and services to support the well-being and development of autistic children.