In light of the growing diversity in U.S. schools, this case study explored how sustained professional development (PD) centered around assets-based approaches can shape the dispositions of teachers of culturally and linguistically diverse (CLD) students. The study followed 84 in-service teachers who completed a 2-year intensive TESOL credentialing course series online that employed the cohort model. The study's purpose was to document changes in in-service teachers' attitudes, knowledge, and practices as they were taught to apply concepts in their practice such as funds of knowledge, cultural wealth, and third space. The results showed significant improvements in teachers' language attitudes, self-assessed teaching abilities, and implementation of culturally responsive teaching practices, as measured by various assessment tools and exit interviews. More specifically, findings indicated that, while teachers overestimated their abilities at the start of the program, they demonstrated substantial growth on key outcomes based on the large effects of their improvement on language attitudes (Cohen's d = 1.30), self-assessed skills and efficacy to teach English learners (Cohen's d = 1.88), and observed teaching practice (Cohen's d = 1.35). Overall, the program effectively shifted teachers from deficit-based to assets-based perspectives on CLD students, enhancing their ability to create inclusive learning environments and utilize students' diverse backgrounds as educational strengths. Interviews further revealed positive program feedback, with teachers praising strategies for family engagement and language support, affirming the transformative power of sustained, community-supported professional development. These findings underscore the importance of ongoing, assets-focused training in teacher education to foster inclusive and effective teaching for CLD learners, supporting a shift in policy and practice towards such models.