Objectives: To determine the prevalence of recent alcohol, nicotine or cannabis use in young persons presenting for mental healthcare. Design: A cross-sectional study of young people seeking mental healthcare completed self-report questionnaires regarding their use of alcohol, nicotine or cannabis. Setting: Data were collected from two sites as part of the national headspace services programme. Participants: 2122 young people aged 12-30 years provided information as part of a patient register; a subset of N=522 participants also provided more detailed information about their patterns of alcohol use. Outcome measures: Prevalence levels of recent alcohol, nicotine or cannabis use within relevant age bands (12-17, 18-19 and 20-30) or primary diagnostic categories. Results: The rates for use at least weekly of alcohol for the three age bands were 12%, 39% and 45%, and for cannabis 7%, 14% and 18%, respectively. The rates of daily nicotine use for the three age bands were 23%, 36% and 41%. The pattern of alcohol use was characterised by few abstainers as well as many risky drinkers. Age of onset across all three substances was approximately 15 years. Individuals who used any of the three substances more frequently were likely to be older, male or have psychotic or bipolar disorders. Conclusions: Frequent use of alcohol, nicotine or cannabis in young people seeking mental healthcare is common. Given the restricted legal access, the patterns of use in those aged 12-17 years are particularly notable. Reductions in substance use needs to be prioritised within services for at-risk young people.