Abstract Background Childhood adversity (CA) is a risk indicator for psychiatric morbidity. Although CA has been linked to violent offending, limited research has considered adolescent psychiatric disorder as a mediating factor. The current study examined whether adolescent psychiatric disorder mediates the association between CA and violent offending. Methods We used a cohort of 476 103 individuals born in 1984–1988 in Sweden. Register-based CAs included parental death, substance abuse and psychiatric disorder, parental criminal offending, parental separation, public assistance, child welfare intervention and residential instability. Adolescent psychiatric disorder was defined as being treated with a psychiatric diagnosis prior to age 20. Estimates of risk of violent offending after age 20 were calculated as incidence rate ratios (IRRs) with 95% confidence intervals (CIs). Mediation was tested with the bootstrap method. Results Exposure to CA was positively associated with violent offending, especially when accumulated. Individuals exposed to 4+ CAs who were also treated for psychiatric disorder had a 12-fold elevated risk for violent offending (adjusted IRR 12.2, 95% CI 10.6–14.0). Corresponding IRR among 4+ CA youth with no psychiatric disorder was 5.1 (95% CI 4.5–5.6). Psychiatric disorder mediated the association between CA and violent offending. Conclusion CA is associated with elevated risk for violent offending in early adulthood, and the association is partly mediated by adolescent psychiatric disorder. Individuals exposed to cumulative CA who also develop adolescent psychopathology should be regarded as a high-risk group for violent offending, by professionals in social and health services that come into contact with this group.