Youth involved in the juvenile legal system face elevated rates of internalizing and externalizing behavioral health problems, including distress, substance use, and antisocial behavior. However, research rarely examines how these problems co-develop and relate to long-term legal outcomes. This study applied group-based multi-trajectory modeling to longitudinal data from 1,216 system-involved male youth (Mage = 15.29; 46% Latino/Hispanic, 37% Black, 15% White, 2% multiracial/other) to uncover patterns of co-occurring externalizing (antisocial behavior, harmful substance use, antagonistic traits) and internalizing (distress) problems and their association with rearrest in young adulthood. We identified eight unique trajectory groups, three of which showed elevated rearrest risk: (1) youth with moderate externalizing and internalizing problems that worsened over time, (2) youth with high-decreasing externalizing problems but moderate-increasing internalizing problems, and (3) youth with high-stable antagonistic traits in the absence of other elevated problems. Membership in these high-risk groups was predicted by specific contextual factors – including peer deviance, violence exposure, negative home environment, and low school orientation – highlighting the role of both risk and protective influences. These findings underscore the utility of longitudinal, dimensional behavioral health assessment in identifying long-term system-involvement risk and tailoring intervention strategies for system-involved youth.