Background: Peer-delivered interventions are a best practice in mental health, yet no such intervention exists for young adults (YA) with intellectual/developmental disabilities with co-occurring mental health conditions. We evaluated YA with intellectual/developmental disabilities' ability to deliver a novel peer mentoring intervention with fidelity and the supports received. Methods: We coded audio-recorded mentoring sessions to evaluate if 4 mentors adhered to 'content' (e.g., psychoeducation) and 'quality' (e.g., validation) fidelity criteria (codes: yes/no). We conducted content analysis of mentor support logs and interviews with mentors', mentors' parents and teachers to describe the supports mentors received. Results: Average fidelity for content criteria (M = 73.3%) was higher than quality criteria (M = 60.0%). Weekly support addressed logistics, delivering content, interpersonal interactions, emotional support, professionalism, and organisation. Family members and teachers rarely provided additional support. Conclusions: With support, YA can deliver a peer mentoring intervention addressing mental health. Additional training activities will be developed to improve fidelity.