In the study reported here, we tested the hypothesis that the Fast Track preventive intervention's positive impact on antisocial behavior in adolescence is mediated by its impact on social-cognitive processes during elementary school. Fast Track is the largest and longest federally funded preventive intervention trial for children showing aggressive behavior at an early age. Participants were 891 high-risk kindergarten children (69% male, 31% female; 49% ethnic minority, 51% ethnic majority) who were randomly assigned to an intervention or a control group by school cluster. Multiyear intervention addressed social-cognitive processes through social-skill training groups, parent groups, classroom curricula, peer coaching, and tutoring. Assigning children to the intervention decreased their mean antisocial-behavior score after Grade 9 by 0.16 standardized units (p < .01). Structural equation models indicated that 27% of the intervention's impact on antisocial behavior was mediated by its impact on three social-cognitive processes: reducing hostile-attribution biases, increasing competent response generation to social problems, and devaluing aggression. These findings support a model of antisocial behavioral development mediated by social-cognitive processes, and they guide prevention planners to focus on these processes.