Using a phase‐in research design, we provide experimental evidence on the impacts of early versus late initiation of iron fortification in school lunch programmes on children's health and cognitive outcomes in India. We find higher haemoglobin levels and a lower likelihood of anaemia in the early treatment group that experienced 4 years of treatment, compared to the late treatment group that was exposed to one and a half years of treatment. Despite significant health gains, we do not find evidence for treatment effects on cognitive and educational outcomes. Heterogeneity analyses show evidence of gendered effects – the anaemia reduction is lower among females relative to males.