This longitudinal investigation assessed whether cognitive schemas of justification of violence, mistrust, and narcissism predicted social information processing (SIP), and SIP in turn predicted aggressive behavior in adolescents. A total of 650 adolescents completed measures of cognitive schemas at Time 1, SIP in ambiguous social scenarios at Time 1 and Time 2, and reactive aggression at Time 1, Time 2, and Time 3 to determine whether SIP measured at Time 2 mediated between the cognitive schemas measured at Time 1 and the aggressive behavior measured at Time 3. The results showed that each schema predicted different SIP components: Justification of violence predicted aggressive response access, narcissism predicted anger and aggressive response access, and mistrust predicted more hostile attributions and less anger. Only the SIP component of aggressive response access was directly associated with reactive aggression. The mediational model was quite similar for boys and girls although some paths, such as the association between aggressive response access and reactive aggression, were higher for boys.