A classroom randomized efficacy trial conducted over four years in 7 community-based preschool and 6 Head Start programs investigated effects of the Second Step Early Learning (SSEL) curriculum on end of preschool executive functioning (EF) and social-emotional (SE) skills in low-income children. Outcomes are reported for n=770 four-year-olds independently assessed for EF and SE by study staff in fall and spring of the prekindergarten year. Main outcomes were analyzed using two, three- level hierarchical linear models, one each for EF and SE skills. A significant effect (effect size of 0.15) for EF and a nonsignificant effect for SE were found. Secondary analyses found no significant differences on pre-academic skills. SSEL appears to have a meaningful impact on at-risk children's EF skills that supports its continued dissemination.