Background: Mental health problems in adolescence are associated with impaired function in young adulthood. Our aim was to assess how a hypothetical reduction in mental health problems in adolescence was related to medical benefits in young adulthood and to examine the mediating role of completion of upper secondary school. Methods: We used a population-based sample of more than 10,000 10th-grade adolescents with self-reported data on internalizing and externalizing mental health problems. The sample was linked to the Norwegian national registers of education and medical benefits. The mediation analysis was based on a causal inference framework. Results: During a three-year period in young adulthood, 6.4% of men and 5.9% of women received medical benefits. A two-point hypothetical reduction in externalizing problems was related to a lower probability of receiving medical benefits of 1.5 (95% confidence interval (CI) 1.0-2.1) percentage points in young men and 1.8 (95% CI 1.3-2.3) percentage points in young women. The proportion mediated by the completion of upper secondary school was 52% (95% CI 36-76) among boys and 42% (95% CI 29-60) among girls. The corresponding reduction in the probability of receiving medical benefits was 1.8 percentage points for internalizing problems in both sexes (95% CI boys 1.2-2.4 and girls 1.4-2.2). The proportion mediated was lower for internalizing problems and was only significant among girls (19%). Conclusions: Intervention and prevention strategies targeting internalizing and externalizing problems in adolescents may have the potential to reduce the receipt of medical benefits in young adulthood. The completion of upper secondary school seems to be a mechanism for this association, especially for externalizing problems.