<p>Research using an undifferentiated measure that mixes mothers’ coparenting conflict with fathers’ has produced mixed results. Moreover, few studies have investigated how mothers’ and fathers’ coparenting conflict affects adolescents’ externalizing problems. Drawing on family systems theory and the coparenting conflict-risk model, this study addressed the issues by exploring the relationship between mothers’ and fathers’ coparenting conflict, mother–adolescent attachment, and externalizing problems, and the moderation effect of prior externalizing problems on this link during early adolescence. Families (<i>N</i> = 340) which included Chinese mothers, fathers, and focal adolescents (<i>M</i>age = 11.28 ± 0.73) participated in a two-wave self-report survey. The results revealed that mothers’ coparenting conflict, but not fathers’ coparenting conflict, predicted adolescents’ externalizing problems (T2), which were fully mediated by maternal attachment (T1). Prior externalizing problems (T1) moderated the direct and indirect association between coparenting conflict and externalizing problems (T2) via maternal attachment. Specifically, the positive link between mothers’ coparenting conflict and externalizing problems (T2) was prominent for adolescents with initially low levels of externalizing problems, supporting the interfering pattern rather than the strengthening pattern. The negative relationship between fathers’ coparenting conflict and externalizing problems (T2) was more salient for adolescents with initially high levels of externalizing problems, supporting the antagonistic pattern. Furthermore, the indirect effect of maternal coparenting conflict on externalizing problems (T2) was stronger for adolescents with higher baseline levels of externalizing problems. These results suggest the differentiated effects of mothers’ and fathers’ coparenting conflict in reaction to initially varying levels of preadolescent behavioral problems.</p>