Objective: Most studies that examine psychiatric illness in people who offend have focused on incarcerated samples, with little known about the larger population of individuals with criminal justice system contact. We examine the overlap between proven offences and psychiatric diagnoses with an emphasis on experiences for Indigenous Australians.

Methods: In a population-based birth cohort of 45,141 individuals born in Queensland, Australia, in 1990 (6.3% Indigenous), psychiatric diagnoses were identified from hospital admissions between ages 4/5 and 23/24 years and proven offences were identified from court records (spanning ages 10–24 years). Prevalence rates for offending, psychiatric diagnoses and their overlap were examined for Indigenous and non-Indigenous individuals. Associations between specific psychiatric diagnoses and types of offending were examined using logistic regressions.

Results: There were 11,134 (24.7%) individuals with a finalised court appearance, 2937 (6.5%) with a diagnosed psychiatric disorder and 1556 (3.4%) with a proven offence and diagnosed psychiatric disorder, with Indigenous Australians significantly overrepresented across all outcomes. Compared with non-Indigenous Australians, Indigenous Australians were younger at their first court finalisation (Cohen’s d = −0.62, 95% confidence interval = [−0.67, −0.57]), experienced a higher number of finalisations (d = 0.94, 95% confidence interval = [0.89, 1.00]) and offences (d = 0.64, 95% confidence interval = [0.59, 0.69]) and were more likely to receive custodial (d = 0.41, 95% confidence interval = [0.36, 0.46]) or supervised (d = 0.55, 95% confidence interval = [0.50, 0.60]) sentences. The overlap between offending and psychiatric illness was more pronounced for Indigenous Australians compared with non-Indigenous Australians (14.8% vs 2.7%). Substance use disorders were the most prevalent psychiatric diagnosis among individuals with a court finalisation (9.2%).

Conclusions: Indigenous Australians were significantly overrepresented in court finalisations and psychiatric diagnoses. Indigenous Australians with a psychiatric diagnosis were at highest risk of experiencing a court appearance, emphasising the importance of culturally appropriate mental health responses being embedded into the criminal justice system.