In the last two decades, a number of Latin American countries expanded preschool availability and made attendance compulsory. In 2002, Mexico launched a reform that mandated three years of preschool before entering primary school, gradually phasing in the requirement. Using nationwide longitudinal administrative educational data, household survey data, and a quasi-experimental regression-discontinuity approach, this paper investigates the medium and longer-term impacts of the mandate. Results show that the preschool mandate enhanced fifth- and sixth-grade math and Spanish scores, improved noncognitive skills, heightened student engagement, reduced failure rates, and led to greater schooling attainment for young adults nearly 20 years post-reform.