Objective: The alliance between patient and therapist is central for therapeutic progress. Social mimicry, the automatic imitation of another person, has been linked to building relationships. This is the first preregistered study to systematically investigate mimicry and its effects on the working alliance in real psychotherapy sessions. Method: Sixty-four different patient-therapist dyads were filmed during a cognitive behavioral therapy (CBT) treatment for obsessive-compulsive disorder (OCD). By using actor-partner interdependence models, we analyzed the extent to which therapists’ and patients’ earlier mimicry predicted their own mimicry (actor effect), how they influenced each other in their mimicry (partner effect) and the effects of mimicry on the patients’ alliance. Videos of 151 sessions were rated by two observers. Results: Patients showed stable low mimicry levels. Furthermore, the patients’ mimicry predicted that of the therapists, indicating that therapists adapt their mimicry levels to patients. Only the patients’ but not the therapists’ mimicry at the beginning of treatment predicted the patients’ alliance. Conclusion: The results suggest that the role of the active patient is underestimated. Furthermore, results offer new treatment targets by shedding light on low mimicry levels displayed by patients, which might lead to social costs.