Background: Children with intellectual disability are less physically active and more sedentary than typically developing peers. To date no studies have tested the feasibility of a school-based walking intervention for children with Intellectual Disability. Method: A clustered randomised controlled trial (cRCT), with an embedded process evaluation, was used to test the feasibility of a school-based walking intervention. Eight schools (n = 161 pupils aged 9-13 years) were randomised into either an intervention arm or an 'exercise as usual' arm. Measures included physical activity, physical fitness and emotional wellbeing. Baseline and 3-month follow-up data were collected. Results: The 'Walk Buds' intervention was found to be acceptable to teaching staff and pupils, with an uptake rate of the walking sessions offered of 84%. Conclusion: A number of challenges were experienced, relating to the COVID-19 pandemic, and difficulties collecting accelerometer data. Barriers, facilitators and required changes identified through the mixed methods process evaluation are discussed.