This paper describes a mixed-methods pilot study that was conducted in collaboration with the developers of a new adult community-building intervention called "Leading Together" (LT), which focuses on strengthening relational trust among staff. The primary research focus of the collaboration was to gather and share descriptive information regarding implementation, and to examine program efficacy. From the context of this work, this study examines how principal and teacher collective efficacy beliefs (CEB) change over the two year arc of implementation of the "Leading Together" (LT) professional development program. Ten intervention schools in the northeastern United States were purposefully sampled by the program designers; two of the schools discontinued participation at the conclusion of the first year. The sample at each school included at least one administrator, one school leader teacher, and one school site teacher who did not participate in training or implementation planning. Teams participating in the direct training and facilitation portion of the intervention included principals and school leader teachers, with each of these LT teams ranging from 3-6 members. This study employs a sequential, exploratory mixed methods design. Quantitative analyses are used to identify changes in CE, as well as the subconstructs group competence and task analysis. Interview data are qualitatively analyzed to explore changes in quantitative levels of CE. Results confirm the CE theoretical model and suggest that implementation of LT may provide opportunities to increase CE levels for participants who carry out the program consistently and as intended. Tables and figures are appended.