This study examined the links between different family structures—capturing type and stability thereof—and preschool-aged children’s likelihood of being obese. We build on the limited number of studies that have pursued this topic by using a large, nationally representative sample of preschool-aged children from the Early Childhood Longitudinal Survey–Birth Cohort (n = 8,350) and exploring a wide range of mechanisms to explain these links. Results revealed that, compared with young children with stably married parents, children in cohabiting- and single-parent families that experienced a prior family structure change were more likely to be obese, except for children in single-parent families born to married parents. Children in step, stably single, and stably cohabiting families were at no greater risk of obesity. These patterns were largely driven by female children, for whom the effects of family structure were most robust. None of the 11 tested mechanisms explained such patterns.