Objective: Therapist adherence is a quality indicator in routine clinical care when evaluating the success of the implementation of an intervention. The current study investigated whether therapist adherence mediates the association between therapist, team, and country-wide experience (i.e. number of years since implementation in the country) on the one hand, and treatment outcome on the other hand. We replicated and extended a study by Löfholm et al. (2014). Method: Data over a 10-year period were obtained from 4290 adolescents (12-17 years) with antisocial or delinquent problem behavior, who were treated with Multisystemic Therapy (MST) by 222 therapists, working in 27 different teams in the Netherlands. Multilevel structural equation modeling was used to assess the associations between experience, therapist adherence, and post-treatment outcomes. Results: Treatment outcomes were directly predicted by therapist experience, countrywide experience, and therapist adherence, but not by team experience. Moreover, therapist adherence mediated the association between therapist and country-wide experience, and treatment outcomes. The association between therapist experience and therapist adherence was not affected by the number of years of team experience or country-wide experience. Conclusion: The effect of country-wide experience on outcome may reflect increasing experience of training and supporting the therapists. It suggests that nation-wide quality control may relate to better therapist adherence and treatment outcome for adolescents treated with systemic therapy.