A key concern among mentoring researchers has been to understand whether mentoring programs can improve outcomes for the most vulnerable groups of youth in society, such as those from families involved with child welfare, juvenile legal, or the criminal legal systems. This chapter synthesizes the evidence regarding youth mentoring as an intervention to support youth involved with these systems, with a focus on research findings and practice insights in relation to each population. Broadly, mentoring interventions can have a positive impact in all three contexts, but programs face challenges in ensuring that youth have access to quality and impactful mentoring support due to the specific nature of the vulnerabilities faced. The chapter concludes with a review of common themes related to mentoring for youth involved with systems, and reflections on how a professional mentoring model could be beneficial in these contexts.