Objective: We assessed sustainability of an empirically supported, transdiagnostic youth psychotherapy program when therapist supervision was shifted from external experts to internal clinic staff. Method: One hundred sixty-eight youths, aged 6–15 years, 59.5% male, 85.1% Caucasian, were treated for anxiety, depression, traumatic stress, or conduct problems by clinicians employed in community mental health clinics. In Phase 1 (2.7 years), 1 group of clinicians, the Sustain group, received training in Child STEPs (a modular transdiagnostic treatment + weekly feedback on youth response) and treated clinic-referred youths, guided by weekly supervision from external STEPs experts. In Phase 2 (2.9 years), Sustain clinicians treated additional youths but with supervision by clinic staff who had been trained to supervise STEPs. Also in Phase 2, a new group, External Supervision clinicians, received training and supervision from external STEPs experts and treated referred youths. Phase 2 youths were randomized to Sustain or External Supervision clinicians. Groups were compared on 3 therapist fidelity measures and 14 clinical outcome measures. Results: Sustain clinicians maintained their previous levels of fidelity and youth outcomes after switching from external to internal supervision; and in Phase 2, the Sustain and External Supervision groups also did not differ on fidelity or youth outcomes. Whereas all 34 group comparisons were nonsignificant, trends with the largest effect sizes showed better clinical outcomes for internal than external supervision. Conclusions: Implementation of empirically supported transdiagnostic treatment may be sustained when supervision is transferred from external experts to trained clinic staff, potentially enhancing cost-effectiveness and staying power in clinical practice.