Introduction: Social behaviour and network therapy involves an active participation of the practitioner in recruiting a supportive network to change the client’s alcohol use. Despite achieving beneficial effects on alcohol consumption, its possible mechanisms of change are a relatively under-studied topic compared to those of other alcohol treatment interventions. This study aimed to explore therapist skills through which social behaviour and network therapy may achieve effects on alcohol consumption in comparison with motivational enhancement therapy. Methods: This study was secondary analysis of data from the UK Alcohol Treatment Trial, a multicentre, pragmatic, randomized controlled trial. The sample comprised 376 participants randomized to motivational enhancement therapy or social behaviour and network therapy. We used the UK Alcohol Treatment Trial Process Rating Scale to assess therapist skills. Outcomes drinks per drinking day and percentage of days abstinent were assessed 12 months after treatment initiation. Analyses were conducted in a simple mediation framework. Results: Therapist skills score (combining frequency and quality) for involving others in behaviour change mediated social behaviour and network therapy effects on percentage of days abstinent (b = 0.06, 95% CI: 0.02; 0.10, p = 0.01). The frequency with which therapists acted as an active agent for change also mediated the effects of social behaviour and network therapy on percentage of days abstinent (b = 0.03, 95% CI: 0.003; 0.05, p = 0.03). The frequency with which the therapist stressed social support as a key factor in achieving change unexpectedly mediated an increase in drinks per drinking day (b = 0.10, 95% CI: 0.01; 0.18, p = 0.02). The two latter mediation effects were not sustained when quality was considered. All other indirect effects tested were non-significant. Discussion/Conclusions: How social behaviour and network therapy exerts effects on alcohol outcomes is not yet well understood and in this study was not attributable to observed ratings of therapist treatment-specific skills. Therapist skill in planning the involvement of others during treatment, however, warrants further study. We suggest that the present findings should be regarded as hypothesis generating as it identifies specific targets for further investigation in alcohol treatment process studies.