Objectives: The objectives of this study were to investigate how family support affects adolescents’ career attitude maturity and to demonstrate the mediating effect of self-efficacy between the two variables. Methods: The researchers collected information from 969 adolescents in Seoul and Incheon, and they used structural equation modeling to analyze relationships among measures of family support, career attitude maturity, and self-efficacy. Results: The hypothesized structural model produced a good fit for the data. Family support had an indirect but important impact on the adolescents’ career attitude maturity, and self-efficacy had significant mediating effects on the relationship between family support and career attitude maturity. Conclusions: The results demonstrate that family support plays a critical role in increasing adolescents’ self-efficacy, and they suggest that as adolescents’ self-efficacy increases, their attitudes toward careers mature. Limitations of the present study and suggestions for future research were discussed.