Public health insurance programs are being expanded across the globe despite limited evidence regarding their impacts on population health. Exploiting the roll-out of Seguro Popular, a largescale program that provides public health insurance to about half Mexico’s population, this research isolates the causal impact of the program on child health and nutrition measured by height-for-age. Drawing on insights from the biology of human linear growth during the first few years of life, we use rich longitudinal population-representative data, the Mexican Family Life Survey, in combination with administrative program data and establish that Seguro Popular has had, at best, a modest impact on child nutritional status. Program effects in a community are larger after the program has been established for several years, suggesting supply-side factors may be critical impediments to fulfilling program goals. The results have important implications for the design, roll-out, and evaluation of public health programs.