Background: Adolescents with Autism Spectrum Disorder (ASD) and intellectual disability evidence significant anxiety. This study aimed to adapt a group cognitive behaviour therapies (CBT) programme designed for youth with ASD and anxiety to meet the cognitive, communication, and behavioural needs of adolescents with intellectual disability, and assess initial feasibility and efficacy of the intervention. Methods: Structural, content and procedural adaptations were made to a 14-week family-focused CBT intervention. Twenty-three adolescents with ASD, intellectual disability and anxiety were included. Treatment acceptability along with adolescent anxiety symptoms was assessed via parent report measures. Results: Of the 23 participants, 19 completed treatment and attended 94% of sessions. Parent acceptability was high. Significant reductions were noted on anxiety symptoms post-intervention. Conclusions: Results indicate that the CBT group was feasible and acceptable. Preliminary outcomes suggest that adolescent anxiety improved, although replication with a larger sample and comparison to a control group is needed.