Most serious mental illnesses (SMI) have onset by emerging adulthood and SMI can impair adolescents' transitions into healthy, productive adults. Emerging adults (EAs) with SMI are at high risk for justice involvement, and rates of recidivism are greater for offenders with SMI than without. These EAs are frequently multi-system involved (e.g., aging out of foster care; both juvenile and adult arrests; prison reentry). Few interventions, however, have focused specifically on EAs, and no interventions have focused on reducing recidivism in EAs with or without SMI. Multisystemic Therapy for Emerging Adults (MST-EA) is an adaptation of standard MST (for adolescent antisocial behavior) that was specifically designed for EAs with SMI and justice involvement. This paper provides the first description of MST-EA, including clinical outcome data on pilot cases and an extensive case example. To date, 57 cases have been treated with MST-EA. Success at discharge was demonstrated on main outcomes (rearrest and mental health) and other functional outcomes. Clinical data on pilot cases is promising and supports further research to assess long-term outcomes and effectiveness.