This article focuses on how length of participation in professional youth work is associated with five outcome variables: prosocial skills, self-mastery, social network, civic participation (volunteering and organizing activities) and finding support from social care services. The study was designed as a longitudinal cohort study of four waves during a 16-month period, gathering the data of 1597 youngsters aged 10–24 who participated in Dutch professional youth work. The results show that, on average, youngsters who participated longer in youth work scored significantly higher on the outcome variables. Respondents did not show individual improvements on outcome variables over the period, but the results demonstrate a cautious positive trend over time in volunteering. Referring youngsters (33%) by youth workers to care services could prevent an exacerbation of existing problems. The results provide knowledge that legitimizes the role of professional youth workers and which can be used by them to improve the support of socially vulnerable youngsters in their personal development and social participation.