Suicide and depression are serious public health issues for adolescents. To increase the effectiveness of interventions for youth depression and suicide, research on the mechanisms of change within existing interventions is an area of increasing interest. Using outcomes data from a large randomized controlled trial and observational coding of therapy sessions, this study addressed whether therapist adherence to core relational interventions in the caregiver–adolescent attachment task of attachment-based family therapy (ABFT) contributes to change in posttreatment outcomes (suicidality, depression, and family functioning). The sample included 58 youth who received ABFT. Trained coders rated 58 taped recordings of the attachment task for each individual using an adherence measure developed for the randomized controlled trial. Findings revealed several associations between therapist adherence and outcomes. First, the attachment task was associated with a reduction in adolescent depression scores posttreatment, at the 95% confidence interval, as well as in suicidality, at the 90% confidence interval. Second, adherence in the first half of treatment (before the attachment task) led to enhanced family cohesion at mid-treatment. Finally, a mediation analysis indicated that greater adherence in the attachment task of ABFT increased family cohesion, which, in turn, improved depression outcomes posttreatment. This study provides support for family cohesion—which, in this study, led to reductions in youth depression symptoms posttreatment—as a mechanism of change in the attachment task of ABFT. Clinical implications, study limitations, and future research are reviewed.