Research consistently demonstrates that common polymorphic variation in monoamine oxidase A (MAOA) moderates the influence of childhood maltreatment on later antisocial behavior, with growing evidence that the “risk” allele (high vs. low activity) differs for females. However, little is known about how this Gene × Environment interaction functions to increase risk, or if this risk pathway is specific to antisocial behavior. Using a prospectively assessed, longitudinal sample of females (n = 2,004), we examined whether changes in emotional reactivity (ER) during adolescence mediated associations between this Gene × Environment and antisocial personality disorder in early adulthood. In addition, we assessed whether this putative risk pathway also conferred risk for borderline personality disorder, a related disorder characterized by high ER. While direct associations between early maltreatment and later personality pathology did not vary by genotype, there was a significant difference in the indirect path via ER during adolescence. Consistent with hypotheses, females with high-activity MAOA genotype who experienced early maltreatment had greater increases in ER during adolescence, and higher levels of ER predicted both antisocial personality disorder and borderline personality disorder symptom severity. Taken together, findings suggest that the interaction between MAOA and early maltreatment places women at risk for a broader range of personality pathology via effects on ER.