Canada, the Netherlands, and the United States differ in their social safety nets, education and labour market systems. This dissertation explores how these differences impact adolescent mental health and entry into higher education and work. The dissertation used population-level cohort data of individuals spanning adolescence to young adulthood. Findings show that across the three countries, adolescents exhibit consistent mental health patterns—with most experiencing low symptom levels over time but some experiencing increasing and/or decreasing symptom frequency—that may be related to physiological and social changes during the transition to adulthood. Higher levels of mental health problems at any point in adolescence are associated with more difficulties in education and work in young adulthood. Additionally, this dissertation suggests that how societies design their social welfare, education, and labour market systems affects the (un)equal distribution of adolescent mental health within a society and the potential for mental health problems to negatively impact education and work in young adulthood. The findings imply that adolescents with mental health problems will have greater success in education and the labour market as young adults if societies more equitably provide economic, educational and employment opportunities. The need for reforms in family, education, and labour market policy is particularly high in the USA. These findings may have implications for recovery from the COVID-19 pandemic given its mental health and labour market impacts. They stress the importance of examining and addressing gaps in the equitable provision of resources to young people with mental health problems who are transitioning to adulthood.