This study explored elementary teachers' (N = 26) development as novice Elementary Mathematics Specialists (EMSs). They worked in 21 urban-situated schools in the United States that served historically marginalised students. Professional learning experiences aimed to prepare them as effective and equitable mathematics teachers and teacher leaders equipped to serve as a more knowledgeable other in their school communities. Their role as EMSs was distinctive because their primary responsibility was teaching students, and their work as a teacher leader was in an informal capacity. A case study design was used, with quantitative and qualitative data forming the descriptive findings. Across two years of professional learning, participants had meaningful improvements in their implementation of learner-centred, equitable mathematics instruction; mathematical knowledge for teaching; and productive mathematics beliefs. Results from interview data illuminated these shifts, with themes related to: "centring of children," "elevating equity and access," "deepening knowledge for teaching," and "navigating constraints in shifting instruction."