Objective: This study aimed to develop a mentalization-based interaction reflection program for early childhood teachers and evaluate its effectiveness. Methods: The program consisted of eight sessions that integrated group activities focused on self-awareness and emotional reflection with individualized video analysis and feedback on teacher–child interactions. To examine the program’s effectiveness, 15 childcare teachers from childcare centers in Busan, South Korea (8 in the experimental group and 7 in the comparison group) completed the Korean Mentalization Scale before and after the intervention. Qualitative data were collected through interviews and analyzed to explore the participants’ reflective experiences throughout the program. Results: First, the experimental group showed significant improvements in overall mentalization and in the other-mentalization subscale compared with the comparison group. Second, the qualitative findings indicated that participants became more aware of their bodily sensations and emotional states, reflected more objectively on their interaction behaviors, and understood how their interaction patterns were related to their emotional states and relational experiences. Participants also demonstrated a shift toward interpreting children’s behaviors not merely as problem behaviors but as expressions of emotions and needs. These findings suggest that changes in teachers’ perspectives occurred through a process whereby self-understanding gradually expanded into a deeper understanding of children. Conclusions: The findings suggest that a mentalization-based interaction-reflection program may enhance early childhood teachers’ mentalization abilities and promote their understanding of both themselves and the children in their care. This study highlights the importance of mentalization-based reflective approaches in early childhood teacher education.