Background: Stepping Stones Triple P (SSTP) is a complex parent-mediated intervention aimed to reduce behaviours that challenge in children with moderate to severe intellectual disabilities, aged 30-59 months. Methods: To formulate a comprehensive understanding of SSTP implementation in the UK, we conducted a process evaluation collecting stakeholder views and considering intervention fidelity, dose, reach, delivery adaptations, and acceptability. Results: Fidelity and quality of delivery ratings were high. Parents perceived SSTP as valuable, reporting increased parental confidence and understanding of the child's behaviours. However, only 30% of families received an adequate dose of the intervention. Parents who only received treatment as usual described feeling abandoned by current services. Service managers emphasised the importance of availability of resources and therapist training for successful intervention delivery. Conclusions: SSTP supports effective management of early-onset behaviours that challenge. Further work is needed to ensure equitable access to the intervention across health and social care services.