In the era of standards-based reforms, informal teacher leadership is a critical factor in realizing instructional improvement. In this paper, we report on data from a one-year study of four early career mathematics teachers engaging in professional development around Common Core mathematical practices and leadership. Our findings highlight how the professional development structure supported the development of early career teachers' leader identity. Through iterative opportunities to participate in two communities of practice (within the professional development setting and in school-based professional learning communities) early career teachers were able to engage in collegial conversations and imagine themselves taking on new roles and responsibilities in order to support the learning of the teachers with whom they worked.