In light of growing social stratification, there have been calls to better understand the developmental antecedents of attitudes and values related to social inequality. In this study we predicted attitudes toward social inequality and social justice values from moral and social antecedents in a representative sample of Swiss adolescents (N = 1,258) at 15 (Time 1), 18 (Time 2), and 21 years of age (Time 3). We assessed children’s sympathy and morals in the context of individuals’ decision-making and anticipation of emotions in moral dilemmas. Social-contextual factors included relationship quality, which was assessed by the quality of one’s closest friendship and education level. Adolescents who reported higher friendship quality and sympathy showed stronger attitudes toward social inequality later. Interestingly, adolescents’ own education level at age 18 positively predicted attitudes toward social inequality at age 21 above and beyond parent education level, but only marginally at a younger age. Social justice values at age 18 were predicted by sympathy and the anticipation of moral emotions at age 15, and social justice values at age 21 were associated with sympathy at age 18. Results are discussed with respect to the potential significance of morality and social-contextual factors in the development of attitudes toward social inequality and social justice values in early adulthood.