Although the promoter polymorphism of the serotonin transporter (5‐HTTLPR) gene is associated with externalizing behavior, its mediating pathways are unknown. Given their sensitivity to serotonin neurotransmission and unique association with attention‐deficit/hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD), we tested callous‐unemotional (CU) traits and narcissism as separate mediators of the association of 5‐HTTLPR with ADHD and ODD. We evaluated 209 5–9 year‐old children with and without ADHD at baseline; approximately 2 years later (i.e., Wave 2), parents and teachers separately rated ADHD and ODD symptoms and youth self‐reported antisocial behavior. Controlling for race‐ethnicity and baseline ADHD/ODD, narcissism uniquely mediated predictions of multi‐informant rated Wave 2 ADHD and ODD from variation in 5‐HTTLPR; CU traits mediated predictions of Wave 2 ADHD from variations in 5‐HTTLPR, but did not mediate the associations of 5‐HTTLPR with ODD or youth self‐reported antisocial behavior. Specifically, the number of 5‐HTTLPR long alleles positively predicted CU traits and narcissism; narcissism was positively associated with Wave 2 ADHD and ODD symptoms, whereas CU traits were positively associated with Wave 2 ADHD. Child sex also moderated indirect effects of CU traits and narcissism, such that narcissism mediated predictions of ADHD/ODD in girls but not boys. Psychopathic traits may represent a relevant pathway underlying predictions of prospective change in ADHD and ODD from 5‐HTTLPR, particularly in girls. We consider the role of psychopathic traits as a potential intermediate phenotype in genetically sensitive studies of child psychopathology. Aggr. Behav. 42:455–470, 2016. © 2016 Wiley Periodicals, Inc.