Abstract Background Adolescence psychiatric and neurodevelopmental diagnoses are common but their link to adulthood over-indebtedness is unknown. This study aims to determine this relationship and explores the possible mediating role of upper secondary education completion. Methods We analyzed the 1987 Finnish Birth Cohort, which consisted of a complete census of children born in Finland in 1987 and registered in the Medical Birth Register (n = 53 743). Records of debt payment defaults, at the age of 33, were used as a measure of over-indebtedness. Adolescent psychiatric and neurodevelopmental diagnoses at ages 13–17 were derived from the national hospital discharge register. Inverse probability treatment weighting was used to investigate the role of pre-exposure variables in this relationship, and the mediating role of upper secondary education completion. Results Compared to unexposed individuals, those affected by an adolescent psychiatric or neurodevelopmental diagnosis had a 15 percentage points higher prevalence of over-indebtedness in adulthood. This association was more common for males and was additionally notably strong for suicidality and conduct and oppositional disorders. Controlling for measured potential confounding factors, the diagnoses were linked to a 11-percentage point (95% confidence interval 9–12) higher risk of over-indebtedness. Completing at least upper secondary education reduced this effect by some 39%. Conclusion People with psychiatric and neurodevelopmental disorders diagnosed in adolescence are at elevated risk of over-indebtedness in adulthood. Recognizing this high risk may help in efforts to prevent further debt problems. Better education may serve as a protective factor against over-indebtedness and perhaps similar other behavioural consequences.