Among children referred to child psychotherapy centers, Oppositional Defiant Disorder (ODD) is considered one of the most common behavioral disorders. Within the framework of developmental psychopathology, ODD and its underlying factors can serve as precursors and predictors of more severe psychological disorders during adolescence and adulthood. Early identification and the application of appropriate therapeutic approaches are critical for reducing current symptoms and preventing long-term complications. The present study aimed to investigate the effectiveness of Mother–Child Mentalization-Based Therapy (MBT) in alleviating ODD symptoms in children aged 5 to 7 years in Zanjan City in 2020. This research employed a single-case study design, and data analysis was conducted using graphical representation, effect size calculation, and improvement percentage indices. From the population of children exhibiting ODD symptoms, four participants were selected through purposive sampling. Pre-intervention and post-intervention evaluations were performed using the Child Behavior Checklist (CBCL) along with structured clinical interviews. The results indicated that Mother–Child MBT led to noticeable symptom reduction for all four participants, with improvement rates of 53% for Participant 1, 67% for Participant 2, 73% for Participant 3, and 53% for Participant 4. Importantly, these therapeutic effects were largely maintained throughout the follow-up period, highlighting the stability of treatment outcomes. Additionally, favorable effect sizes were observed in all cases, suggesting robust clinical impact of the intervention. Taken together, these findings demonstrate that Mother–Child Mentalization-Based Therapy is an effective approach for reducing symptoms of ODD in young children, offering the dual benefit of improving current functioning while preventing future psychological difficulties, and thereby contributing significantly to the promotion of children’s long-term mental health and well-being.