Background: Psychological interventions need to be adapted for use with people with intellectual disabilities to ensure they are engaging, accessible and effective. Co-design allows the experiences of service users and stakeholders to actively shape and develop interventions, to ensure their accessibility. Method: An adapted model of Experience Based Co-Design (EBCD) was used to co-develop a novel, mental imagery-based psychological intervention for people with mild to moderate intellectual disabilities and anxiety. Involvement in EBCD was evaluated for people with intellectual disabilities and stakeholders using both quantitative and qualitative methods. Results: Numerous concrete and specific intervention adaptations arose and were implemented. Our findings indicated that all participants were able to engage fully with EBCD, and that participants found the process a positive experience. Conclusions: EBCD has likely resulted in a more accessible and engaging intervention which can be now tested within a larger study.