This study attempted to verify the mediating effect of mentalization by mothers in the influence of the mothers’ insecure adult attachment on the problematic behavior of their early-school-age children. To this end, data were collected through an online survey of 390 mothers of elementary school children in the first, second, and third grade nationwide. As a result of the study, first, the direct path through which the mothers’ attachment avoidance affects internalization and externalization problem behavior by their children was not significant. Second, it was confirmed that the mothers’ mentalization had a significant effect while completely mediating in the relationship between attachment avoidance and the child’s internalization problem behavior. Third, it was confirmed that the mothers’ attachment anxiety had a direct and significant effect on their children’s internalization and externalization problem behavior. Fourth, in the relationship between attachment anxiety and children’s externalization problem behavior, a mediating effect of mothers’ mentalization was confirmed, but for internalization problem behavior, a mediating effect of mentalization could not be confirmed. The results of this study suggest that if a child’s problem behavior occurs and the mother’s attachment is judged to affect the problem behavior at this time, different approaches can effectively mitigate the problem after the attachment type is ascertained.