Background: This study examines participant satisfaction and effectiveness of the online mindset intervention 'The Growth Factory' (TGF) for youth with intellectual disabilities using a randomised controlled trial design. Method: Youth with mild to borderline intellectual disabilities (N = 119; 12-23 years) were randomly assigned to TGF (n = 60) or control group (n = 59). Primary outcome measures were mindsets and perseverance. Secondary outcomes were empowerment, mental health problems, self-esteem, treatment motivation, therapeutic alliance and challenge seeking. Measurements were conducted at pre-test, post-test and at 3 and 6 months follow-up. Results: TGF had positive effects on perseverance, mental health problems, self-esteem and therapeutic alliance at post-test. TGF had follow-up effects on mental health problems (3 months), mindset of intelligence (3 and 6 months) and mindset of emotion and behaviour (6 months). Conclusions: TGF offers a promising add-on intervention complementing usual care programmes accelerating improvements in mindsets and mental health in youth with intellectual disabilities.