Introduction: This study investigated substance use and help-seeking among justice-involved young people to inform and improve service provision during and after contact with the justice system. Methods: Young people (14–17 years) in the community with current or prior contact with the justice system were recruited in Queensland and Western Australia, Australia using purposive sampling between 2016 and 2018. A cross-sectional survey was delivered by computer-assisted telephone interview. Information was collected on sociodemographic and health factors; lifetime and frequency of use of alcohol, tobacco and other drugs; and use of health services related to substance use and mental health. Results: Of the 465 justice-involved young people surveyed, most had used alcohol (89%), tobacco (86%) or other drugs (81%). Of the latter, cannabis use was most prevalent (79%), followed by ecstasy (26%) and amphetamine (22%). Young people engaging in higher risk drug use (daily use, injecting use) were more likely to also have an alcohol use disorder, be disengaged from education, unemployed, have attempted suicide and experienced incarceration. Of the cohort, 24% had received treatment at an alcohol and drug service in the past year and 30% had seen a health professional about emotional/behavioural problems. Males and Aboriginal and Torres Strait Islander young people were less likely to have sought professional help. Conclusion: The high levels of substance use and disproportionate levels of help-seeking observed in this study illustrate the importance of delivering tailored, comprehensive and coordinated trauma-informed and culturally safe alcohol and drug services to justice-involved young people.