The study examined potential mediating effects of therapist behaviors in the per-protocol sample (n = 108) of a randomized controlled trial comparing a behavioral and a nondirective guided self-help intervention for parents of children with externalizing disorders (4–11 years). Additionally, from an exploratory perspective, we analyzed a sequential model with parental adherence as second mediator following therapist behavior. Outcomes were child symptom severity of attention-deficit/hyperactivity disorder (ADHD) and oppositional defiant disorder rated by blinded clinicians, and parent-rated child functional impairment. We found a significant indirect effect on the reduction of ADHD and functional impairment through emotion- and relationship-focused therapist behavior in the nondirective intervention. Additionally, we found limited support for an extended sequential mediation effect through therapist behavior and parental adherence in the models for these outcomes. The study proposes potential mediating mechanisms unique to the nondirective intervention and complements previous findings on mediator processes in favor of the behavioral group. Trial registration ClinicalTrials.gov NCT01350986.