Previous research has consistently documented high rates of tobacco smoking and substance use disorders among young people with serious mental illness. However, limited studies have been conducted outside traditional clinical settings. In an attempt to address this shortfall and to better understand the needs of young people accessing its services, a national psychiatric disability support service (Neami) examined the prevalence and impact of substance use among 120 consumers from across Australia aged 30 years and under. Results indicated extremely high rates of tobacco use, with 68% of participants identified as daily smokers. In addition, 33.4% met criteria for an alcohol use disorder, whilst 40.8% met criteria for a cannabis use disorder. Overall rates of other illicit substance use were low. This study reinforces the need for routine alcohol and drug screening and assessment across all mental health settings, as well as appropriate support strategies that address high level smoking and caffeine intake.