BACKGROUND Comparative literature investigating race/ethnic patterning of children’s health has found racial/ethnic minority status to be linked to health disadvantages. Less is known about differences during early adolescence, a period during which health outcomes are linked to later life health. OBJECTIVE Using the UK Millennium Cohort Study (n = 10,188) and the US Early Childhood Longitudinal Survey–Kindergarten Cohort (n ~ 6,950), we examine differences in socioemotional and cognitive development among 11-year-old adolescents and the contribution of family resources in explaining any observed differences, including socioeconomic, cultural traditions, and psychosocial resources. RESULTS Adverse socioemotional health and cognitive development were associated with race/ethnic minority status in both countries. In the United States, we found that cultural resources and family socioeconomic capital played a large role in attenuating differences in problem behaviors between Asian American, Black, and Latino adolescents and their White peers. In the United Kingdom, the explanatory factors explaining differences in problem behaviors varied by racial/ethnic group. In both contexts, family resources cannot explain the sizable cross-country differences in verbal skills. In the United Kingdom, Indian adolescents had nearly one-third of a standard deviation increase in their verbal scores whereas in the United States, Black and Latino adolescents had scores nearly two-fifths and one-fifth of a standard deviation below the mean, respectively. CONTRIBUTION We use a detailed race/ethnic classification in the investigation of racial/ethnic inequalities across the United States and United Kingdom. There are strong family resource effects, suggesting that relative family advantages and disadvantages do have meaningful associations with adolescent socioemotional and cognitive development. Although levels of resources do explain some cross-national differences, there appears to be a broader range of family background variables in the United Kingdom that influence adolescent development. Our findings point to the critical role of both the extent and nature of family social capital in affecting adolescent development.