Research Findings: This study examined the extent to which maternal sensitivity in infancy and toddlerhood is associated with children's social and relational competence and problems in the early years of schooling as well as the extent to which this association is mediated by children's effortful control abilities. Data from 1,364 children (705 boys, 659 girls), their mothers, and teachers from the longitudinal National Institute of Child Health and Human Development Study of Early Child Care and Youth Development were used. Maternal sensitivity was assessed by coding semistructured videos of mother–child interactions; effortful control was assessed by maternal report; and children's social competence, problems, and relationships with teachers and peers were assessed by school observations and teacher report. Structural equation models examined the extent to which there was an association between maternal sensitivity and children's social and relational competence and problems as well as the extent to which this association was mediated by children's effortful control skills. Maternal sensitivity had a direct association with children's social and relational competence and problems in 1st grade. Children's inhibitory control partially mediated the association between maternal sensitivity and the quality of children's skills. Practice or Policy: The results are discussed in terms of the importance of interventions geared toward improving maternal sensitivity and children's effortful control skills to help children develop better social and relational skills to foster close and supportive relationships with teachers and peers.