Research Findings: The purpose of this study was to test the premise that children's effortful control (EC) is prospectively related to their academic achievement and to specify mechanisms through which EC is related to academic success. We used data from 214 children (M age at Time 1 [T1] = 73 months) to test whether social functioning (e.g., social competence and externalizing problems) mediated the relations between EC and academic achievement. Children's adult-reported and observed EC were assessed at T1. Parents' and teachers' reports of social functioning were obtained 2 years later (T2), whereas teachers' and children's reports of academic achievement were obtained 4 years after T2 (T3). Children's T2 social functioning fully mediated the relation between T1 EC and T3 academic achievement in a structural equation model. Practice or Policy: Findings highlight the importance of considering social and emotional processes when attempting to improve academic achievement and have implications for curriculum developers and professionals working in preschool programs and elementary schools. (Contains 1 figure, 1 table and 1 footnote.)