It is unclear how individual differences in parenting and brain development interact to influence adolescent mental health outcomes. This study examined interactions between structural brain development and observed maternal parenting behavior in the prediction of adolescent depressive symptoms and psychological well-being. Whether findings supported diathesis-stress or differential susceptibility frameworks was tested. Participants completed observed interactions with their mothers during early adolescence (age 13), and the frequency of positive and aggressive maternal behavior were coded. Adolescents also completed structural magnetic resonance imaging scans at three time points: mean ages 13, 17, and 19. Regression models analyzed interactions between maternal behavior and longitudinal brain development in the prediction of late adolescent (age 19) outcomes. Indices designed to distinguish between diathesis-stress and differential susceptibility effects were employed. Results supported differential susceptibility: less thinning of frontal regions was associated with higher well-being in the context of low levels of aggressive maternal behavior, and lower well-being in the context of high levels of aggressive maternal behavior. Findings suggest that reduced frontal cortical thinning during adolescence may underlie increased sensitivity to maternal aggressive behavior for better and worse and highlight the importance of investigating biological vulnerability versus susceptibility.