The current study tested whether teacher-rated externalizing behavior and academic (reading) performance mediate the relationship between childhood onset conduct disorder and self-reported adolescent delinquency and officially recorded adult offending. All 411 boys from the Cambridge Study of Delinquent Development served as participants in this study. Mediation analysis revealed that the direct effect of childhood onset conduct disorder and the indirect effect of reading performance on mid-adolescent delinquency and early adult offending were nonsignificant, but that adolescent externalizing behavior had a significant indirect effect on both delinquency and adult offending.