Reducing social assistance dependency is high on the political agenda; labour market and social exclusion of youth, in particular, has been considered worrisome. With these policy objectives and societal considerations in mind we set to study the association between health problems, with a specific focus on mental health, and the duration of social assistance receipt among young adults in Finland. Our analyses are based on rich register data encompassing the total population in the metropolitan area of Finland from 2005 to 2016. We follow a cohort of new social assistance recipients (n=36,728) aged 18–34 for a maximum of 60 months using Cox proportional hazard model. The results show a strong association between mental health problems and duration of social assistance receipt. The association was the strongest among those aged 18–24. Some differences in the strength of the association were found for different psychiatric diagnoses. On the other hand, somatic diagnoses were only weakly associated with duration of social assistance receipt. Mental health problems appear to be an important obstacle for exiting social assistance and this should be taken into account when governments aim to decrease welfare dependency and reform the last-resort financial aid.