Justice-involved youths are more likely to have mental health problems than peers in the community. Therefore, it is important to develop an understanding of the antecedents of mental health problems in this group. The present study examined the association between childhood trauma and mental health problems in juvenile justice-involved adolescent males (N = 422), comparing childhood-onset with adolescent-onset offenders. Childhood-onset offenders were more likely than adolescent-onset offenders to report mental health and substance use problems, as well as childhood maltreatment. Via structural equation modeling, we found that childhood trauma predicted mental health problems in both offender groups. Multigroup analysis revealed a moderation effect of offender group: The association between trauma and mental health problems was stronger in adolescent-onset offenders than in childhood-onset offenders. Thus, mental health problems were more prevalent in childhood-onset offenders, but these problems were less well-explained by childhood trauma in childhood-onset than in adolescent-onset offenders. Theoretical and practical implications are discussed.