Objective: Measurement-based care (MBC), which collects session-by-session symptom data from patients and provides clinicians with feedback on treatment response, is a highly generalizable evidence-based practice with significant fi cant potential to improve the outcomes of mental health treatment in youth when implemented with fi delity; however, it is rarely used in community settings. This study tested whether an implementation strategy targeting organizational leadership and organizational implementation climate could improve MBC fi delity and clinical outcomes for youth in outpatient mental health clinics. Method: In a cluster randomized trial, 21 clinics were assigned to the Leadership and Organizational Change for Implementation strategy plus training and technical assistance in MBC (k = 11, n = 117) or training and technical assistance only (k = 10, n = 117). Primary outcomes of MBC fi delity (assessed via electronic metadata) and youth symptom improvement (assessed via caregiver-reported change on the Shortform Assessment for Children Total Problem Score) were collected for consecutively enrolled youths (ages 4-18 years) who initiated treatment in the 12 months following MBC training. Outcomes of each youth were assessed for 6 months following baseline. Results: A total of 234 youths were enrolled and included in intent-to-treat analyses. At baseline, there were no significant fi cant differences by condition in clinic, clinician, or youth characteristics. Youths in clinics using the Leadership and Organizational Change for Implementation strategy experienced significantly fi cantly higher MBC fi delity compared with youths in control clinics (23.1% vs 3.4%, p = .014), and exhibited significantly fi cantly greater reductions in symptoms from baseline to 6 months (d= = 0.31, 95% CI: 0.04-0.58, p = .023). Conclusion: Implementation strategies targeting organizational leadership and focused implementation climate can improve fi delity to evidence-based practices and clinical outcomes of youth mental health services. Plain language summary: Measurement-based care is an evidence-based intervention that has been shown to improve the outcomes of mental health services in dozens of randomized trials; however, it is not widely used in routine practice. In this randomized controlled trial of 21 outpatient mental health clinics and 234 youth, the authors found an organizational intervention which trained clinic leaders in skills to support the use of measurement-based care increased the extent to which patients received measurement-based care compared to control clinics where clinic leaders did not receive this training (23% vs. 3%). Youth receiving services in the clinics that received the organizational intervention also had greater reductions in mental health symptoms. Clinical trial registration information: Working to Implement and Sustain Digital Outcome Measures (WISDOM); https://clinicaltrials.gov/; NCT04096274.