Objective: The present study aimed to characterise the intersecting sociodemographic, psychiatric and substance use needs of young people seeking treatment across multiple primary mental health, justice and alcohol and drug services within Australia. Methods: Data from four separate studies (N = 867) that investigated social and emotional wellbeing in young people aged 16-25-years-old were used in the present study (three studies recruited service users; one study recruited community participants) Results: The sociodemographic characteristics of service users differed substantially to the community sample. Service users identified as non-binary, LGBTIQ+ and Aboriginal and/or Torres Strait Islander at substantially greater proportions compared to the community sample. Family violence, involvement in criminal justice, homelessness and child protection services was considerably more common among service users than in the community sample and nearly one-third of service users were disengaged from opportunities for learning and vocational attainment. With respect to psychiatric characteristics, the majority of service users had been diagnosed with two or more psychiatric disorders and, almost two- thirds reported engagement in non-suicidal self- injury across their lifetime. Service users engaged in lower rates of lifetime and harmful alcohol use compared to the community sample; however, they reported significantly higher rates of lifetime and harmful drug use with more than one-third of service users engaged in poly-substance use. Conclusion: The comprehensive examination of the sociodemographic, psychiatric and substance use characteristics of young service users in the present study highlights the intersection of early childhood adversity, adolescent mental ill-health, polysubstance use and psychosocial vulnerabilities across multiple determinants of social and emotional wellbeing. The cohort were indeed representative of some of the most vulnerable in our society. The need for developmentally informed, inter-sectoral systems of care that address the intersecting vulnerabilities of these young Australian service users are discussed