Despite youth being the most common age group for onset of mental disorders, there is less knowledge on the benefits and harms of treatments in young people. In addition, efficacy data from randomized controlled trials may not generalize to how treatment works outside of research settings. This study aimed to investigate young people's perceived effectiveness of different treatments for mental health problems, the professionals who delivered these, and the experience of negative effects. We developed a consumer report website where young people who were ever diagnosed with a mental disorder provided ratings on the helpfulness or harmfulness of different types of professionals, mental health treatments (medical, psychological complementary/alternative) and self-help strategies, and whether they had experienced particular negative effects. Here, 557 young people aged 12-25 years, who were recruited from English-speaking, high-income countries, provided 1258 ratings of treatments. All treatments showed varied perceptions of effectiveness. Medical and psychological treatments were rated moderately helpful on average with low rates of harmfulness. Self-help strategies were rated as being as helpful as professional treatments. Side effects related to the head or mind (e.g., concentration difficulties, inability to feel emotions, depression and irritability) were the most common across all types of medicines. For psychological treatments, treatment being too expensive and feeling worse at the end of a session were the most commonly reported negative effects. Study findings may be a useful guide to clinicians, researchers, young people and their families about what is likely to work in real-world settings. © 2020 John Wiley & Sons Australia, Ltd.