A challenge for public schools is to successfully support and professionally develop early career teachers (ECTs) and thereby prepare them for long and successful careers in education. The purpose of this qualitative research study was to describe how the professional practices of early career science and mathematics teachers, some of whom are career changers, were influenced by their interactions with mentors and professional communities. Topics examined included the contextual elements that influenced the ECTs' interactions with mentors and professional communities, how teachers positioned themselves within multiple professional communities, and how they perceived these experiences had influenced the development of their teaching practice. An extensive semi-structured interview of the ECTs generated data that were analyzed to identify emergent themes and patterns. The findings of this study indicated that navigating professional communities and interacting with mentors had influenced the ECTs' decisions to adopt important components of a learner-centered approach to teaching that included engaging students in active learning processes, utilizing formative assessment, and responding to students' individual needs. These findings have implications for school policies and approaches related to supporting and professionally developing unique cohorts of ECTs.