More than 500,000 US citizen migrant children resided in Mexico in 2015, and more than half of them had limited and inadequate health insurance despite their citizenship status. The majority of these children lived in Mexican states near the US border. Despite these figures, knowledge about these children and their health is scarce. To address these knowledge gaps, we analyzed data from the 2015 Mexican Intercensal Survey to examine whether the health insurance status of migrant U.S. citizen children in Mexico is related to individual, family, and state factors. We compared the rates of insured US citizen migrant children with the rates among those who were underinsured. We have found high rates of underinsurance among migrant children who are US citizens, particularly in border states in northern Mexico. Parents’ education, participation in the labor market, urban residence and border residence partly explain the probability of these children to be insured. Our findings have implications for binational policies that extend health care protection to migrant U.S. citizen children through reintegration assistance for their parents, an expedited dual citizenship application process, and the exemption of such children from the ‘Automatic cancellation of US-based health benefits.