The involvement of people with serious mental illness (SMI) with the justice system may be a direct result of their disruptive/unsafe expression of psychiatric symptoms being responded to by law enforcement. SMI may also indirectly contribute to justice involvement, through exposure to environmental and social learning processes that place people with SMI at risk for criminal behavior. This study addresses the question: For whom does SMI directly or indirectly relate to criminal behavior? Mediation and conditional effects testing were used to examine the potential of early onset of criminal behavior to distinguish those groups for whom SMI displays a direct effect or an indirect effect on criminal recidivism. This study utilized a disproportionate random sample of 379 inmates released from New Jersey Department of Corrections; 190 of whom had SMI and 189 of whom did not have SMI. Data were collected from clinical and administrative records. Results indicate that criminal risk mediated the relationship between SMI and recidivism. This indirect effect was conditioned by whether the individual had a juvenile conviction. Specifically, for early start offenders, criminal risk was positively related to recidivism while this relationship was not observed for late start offenders. Juvenile criminal onset did not condition the direct effects of SMI on recidivism. A juvenile history of criminal involvement may signal the presence of heightened criminogenic need among adults with SMI. This simple indicator could function to differentiate for clinicians those adults who are good candidates for exploring further, and targeting for amelioration, criminogenic needs to reduce further criminal involvement.