We used data from the National Longitudinal Study of Adolescent Health (N = 7,686) to determine whether racial and ethnic differences in socioeconomic stress and social protection explained group differences in the association between family structure instability and three risk behaviors for White, Black, and Mexican American adolescents: delinquent behavior, age at first nonmarital sex, and age at first nonmarital birth. The positive association between mothers' union transitions and each outcome for White adolescents was attenuated by social protection. The association of instability with age at first sex and first nonmarital birth was weaker for Black adolescents but not for Mexican American adolescents. The weaker association was explained by Black adolescents' more frequent exposure to socioeconomic stress in the context of union instability.