["Oxford Bulletin of Economics and Statistics, EarlyView. ", "\nABSTRACT\nThis paper analyses the long‐term effects of preschool education in France. We focus on the effect of preschool duration on educational attainment and labour market earnings in adulthood. Building on an old tradition of free and universal pre‐primary education, France implemented in the late 1960s a large‐scale expansion of preschooling facilities, leading to a massive increase in the supply of preschool. We rely on the independent variation across counties and cohorts in preschool supply, induced by this policy, to assess the effect of preschool using a Two‐Sample Two‐Stage Least Squares estimator. While the preschool expansion increased preschool duration (+0.69$$ +0.69 $$ years per additional school created), we find on average small and non‐significant long‐term effects on employment and earnings, with some positive impacts on educational attainment. We also investigate heterogeneity by parental socio‐economic status and find that children from an advantaged family background benefit significantly more in terms of both education and earnings than their peers from less advantaged backgrounds. One additional year in preschool thus increases the achievement gap for most outcomes including the likelihood of graduating from high school (+8.3$$ +8.3 $$ percentage points) or college (+13.7$$ +13.7 $$ pp), employment (+13.3$$ +13.3 $$ pp), unemployment (−6.4$$ -6.4 $$ pp) and earnings (+5.4%$$ +5.4\\% $$).\n"]