Professional learning communities (PLCs) support the growth of educators to improve learning outcomes for all students. Guided by social constructivist and social cognitive theories, this longitudinal participatory action research study explored the implementation of an interdepartmental PLC across Hord’s five dimensions of a successful PLC—supportive and shared leadership, shared beliefs values and vision, collective learning and its application, shared personal practice, and supportive conditions. Additionally, this study explored how engaging with an interdepartmental PLC influenced participants’ collective efficacy. During the PLC, the collective expertise of mathematics teachers, administrators, and additional educational specialists (such as occupational therapists and speech and language pathologists) (n = 13) were leveraged to develop strategies for the improvement of mathematical problem solving among elementary and middle school students with disabilities. Our findings indicate statistically significant increases in the five dimensions of the PLC over time, with qualitative evidence supporting the PLC’s effectiveness. However, our findings revealed no significant increase in participants’ overall collective efficacy, a group’s shared belief that together they can achieve a desired result. Implications for practice and research are discussed.