This research examines the prevalence of adverse childhood experiences (ACEs) in a representative sample of young people under youth justice supervision in South Australia. Data were collected from a set of self-report assessments administered to 184 young people, which were subsequently linked to administrative records. The analysis showed that not only was the prevalence of ACEs particularly high in this population (89% experienced a combination of maltreatment and household dysfunction), but so too were trauma symptomatology, substance use, and internalising and externalising behaviours (with more than two-thirds of young people scoring in the clinical ranges on each of these measures). Using latent class analysis, four distinct subgroups of young people were identified according to different patterns of ACEs experienced. These findings offer empirical evidence for youth justice policymakers and practitioners to develop new ways of working with justice-involved young people that take account of key developmental experiences of adversity.