In theory, misalignment of education and career aspirations in high school is a crucial determinant of post-school education and career mismatch. However, existing scholarship investigates the social influence and rational choice determinants of misalignment and mismatch separately. This study examines how misalignment at 15 years of age correlates with career mismatch in young adulthood (25 years). Using data from Y09 cohort in the Longitudinal Surveys of Australian Youth (LSAY), it analyses the education and career aspirations of 7,871 15-year-olds and the education and career outcomes of 2,697 of the same cohort after ten years. Consistent with both social class influences and rational choice factors, the results show that young people who were female, misaligned at school, in vocational pathways, were in relatively poor health and wealth when growing up and had a father with lower levels of education were more likely to be mismatched. Notably, misalignment of career and educational aspirations at age 15 was the strongest indicator of employment outcomes in early adulthood at age 25. The findings urge a rethinking of national and state investment in careers education, especially in addressing disadvantages faced by equity groups and within occupations served by vocational education.