Preschool programs can have lasting effects on education and labor market outcomes, but we have limited knowledge of what skills these programs should target. Executive functions at start of schooling predict academic success, yet we do not know whether preschool-program-induced improvements in executive functions boost achievement in primary school. By combining experimental data with a skill-building model, we find that preschool-program-induced improvements in executive functions boosted language and mathematical skills in primary school. One reason for this could be program-induced improvements in executive functions enable children to derive greater advantages from primary school investments (i.e., dynamic complementarity).