Research shows that parenting plays an important role in the development of callous-unemotional (CU) traits in children. Yet, the specific aspects of positive parenting that may offer the strongest protection against the development of CU traits, as well as the potential role of child attachment to parent in this protection, remain poorly understood. This longitudinal multi-informant study aimed to investigate the mediating role of early mother–child attachment security in the prospective associations between three aspects of maternal sensitivity (positivity, attunement, availability) and subsequent CU traits in children. Maternal sensitivity and mother–child attachment security were observed in the home when children were 12 and 15 months old respectively. Child CU traits were reported by mothers, fathers, and teachers at age 4 years. Analyses revealed that maternal attunement was linked to lower levels of CU traits indirectly through the mediating role of attachment security. There was also a direct, non-mediated negative association between maternal availability and CU traits. Consistent with the notion of equifinality, these findings suggest that different aspects of parenting may be linked to child CU traits via distinct mechanisms, with some but not all of those mechanisms involving parent–child attachment.