Juvenile offenders may be too young to manage the terms of their probation independently; a parent's participation in the probationary process is critical for youths' successful transition to crime desistence. However, a parent's capacity for support during his or her child's legal process may depend on the parent's knowledge of how the justice system operates. The present study is the first to quantify mothers' knowledge of the juvenile justice system. The authors examine the association between mothers' legal knowledge, legal participation, and youth re-offending using a longitudinal sample of 324 dyads (total N = 648) of mothers and their sons, all first-time juvenile offenders. Results indicate that mothers averaged a 66% out of 100% on a test of legal knowledge. Importantly, those mothers who knew the least about the system also participated the least in their son's legal process, and mothers who participated the least had sons who self-reported re-offending more within the first year after his arrest. Practitioners are encouraged to educate parents of juvenile offenders about their rights and responsibilities in the courtroom and on probation, to create meaningful avenues for parental involvement, and to set youth up for success after a first arrest.