This paper analyzes the long-term effects of preschool education in France. We focus on the effect of preschool duration on educational attainment and labor market earnings in adulthood. Building on a old tradition of free, universal pre-primary education, France implemented in the late 1960s a large scale expansion of facilities, leading to a massive increase in the supply of preschool. We rely on exogenous variations across counties and cohorts in preschool supply, induced by this policy, to assess the effect of preschool duration on long-term outcomes, using a Two-Samples Two-Stages Least Squares estimation. While the preschool expansion significantly raised attendance and duration, we do not find, on average, significant long-term effects on education and earnings. We also investigate heterogeneous effects by parental socio-economic status and find that children from advantaged family background significantly gained in terms of both education and earnings, compared to lower background children.