<p>Although prior research suggests that executive functioning may either mediate or moderate the association between family socioeconomic status (SES) and children’s problem behaviors, examining these roles separately provides an incomplete account: mediation models may understate individual differences that are not attributable to the environment, whereas moderation models may understate the role of the environment in shaping personal characteristics. To integrate these perspectives, the present longitudinal study examined whether executive functioning simultaneously mediates and moderates the association between family SES and children’s internalizing and externalizing problems. A total of 308 children in the early years of elementary school (<i>M</i>_ageT1 = 7.34 years; 138 girls) were assessed and followed up 39 months later. After controlling for children’s gender, age, and grade, lower family SES at T1 significantly predicted higher levels of both internalizing and externalizing problems at T2. Executive functioning at T1 partially mediated these associations, indicating that differences in children’s executive functioning partly accounted for socioeconomic disparities in problem behaviors. Executive functioning also moderated both associations: SES was negatively associated with problem behaviors among children with lower executive functioning but not among those with higher executive functioning. These findings highlight the dual role of executive functioning in the longitudinal association between SES and children’s problem behaviors and suggest that executive functioning may be a promising target for efforts to reduce mental health disparities associated with socioeconomic disadvantage.</p>