We investigated teacher learning within a professional development (PD) workshop series on computational thinking (CT) for elementary-level mentor teachers. The purpose of the PD was to prepare mentor teachers to support preservice teachers in integrating CT into their classroom practice, toward the broader goal of advancing CT for all in the early grades. We examined the ways in which participants collaboratively built on existing professional knowledge as they engaged in professional learning activities designed to introduce CT and related pedagogies for elementary science education. Our data sources were field notes, artifacts, drawings, written reflections, and focus group interviews. We describe how participants developed new understandings of CT integration and made connections to existing professional knowledge of their students, their curriculum, and their school contexts. We discuss implications for teacher learning and PD design relevant to CT, and make recommendations for future research.