Recent research has brought attention to the hardship faced by children of immigrants in the United States, particularly in the Mexican-origin population. In this study, the authors are concerned with the extent to which U.S.-born children of Mexican immigrants who live in unmarried families may face exceptional risks. Using data from the Fragile Families and Child Wellbeing Study, the authors find that young children of Mexican immigrants in unmarried families face significant disadvantages on a variety of levels compared with children of U.S.-born mothers. Mexican immigrant mothers have significantly lower levels of education and employment and much higher rates of poverty, as well as less access to social services. Although characterized by low rates of low birth weight and more positive maternal health behaviors, their poor socioeconomic and social service profile suggests that even when healthy at the starting gate, they may potentially face poor outcomes during childhood and beyond. (Contains 5 tables.)