Teacher leadership is a critical factor in realizing instructional improvements aimed at providing equitable opportunities for all learners. In this paper, we report on data from a one-year study of early career mathematics teachers engaging in professional development (PD) around Common Core mathematical practices and leadership. We drew on Wenger's (1998) conceptualization of identity to explore how reification and co-participation in leader PD and school-based PLCs provided opportunities for engagement, imagination and alignment. Our findings highlight how the PD structure, coupled with building level support from both mathematics coaches and administrators supported the development of teachers' leadership identity.