Background: Individuals with attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) can experience obstacles in traditional health care situations due to difficulties associated with their impairment. Objective: This controlled study aims to investigate the feasibility of an internet-based support and coaching intervention (IBSC), including 2 weekly chat sessions and 2 complementary clinic visits with coaches over the course of 8 weeks, for adolescents and young adults with ADHD and/or ASD in 2 naturalistic routine care settings. Methods: Individuals with ADHD and/or ASD aged 15-32 years were recruited in 2 clinical settings, where they received either IBSC (n=24) or treatment as usual (TAU; n=20). Outcome measures included self-report questionnaires assessing quality of life (Manchester Short Assessment for Quality of Life), sense of coherence (Sense Of Coherence 29), self-esteem (Rosenberg Self-Esteem Scale), and anxiety and depressive symptoms (Hospital Anxiety and Depression Scale [HADS] and Montgomery-Åsberg Depression Rating Scale-Self-reported, respectively). Results: Significant between-group effects were observed in measures of anxiety (HADS) at postintervention (P=.02) as well as at the 6-month follow-up (P=.004). Significant between-group effects were also noted for depressive symptoms (HADS) postintervention (P=.04). The between-group effects were partially explained by a deterioration in the TAU group. A significant increase in self-esteem (P=.04) as well as a decrease in anxiety (P=.003) at the 6-month follow-up was observed in the intervention group following IBSC. Findings from a qualitative study of the intervention are consistent with the results. Conclusions: The findings from this study suggest that IBSC holds promise as a feasible complement or alternative to traditional face-to-face health care meetings.