Supporting the growth and retention of teachers in urban classrooms poses significant challenges to the education community. Student-centred instruction has demonstrated positive impacts on student academic growth and social development, facilitating acquisition of 21st century skills. However, implementing student-centred instruction is demanding for teachers, requiring high quality professional development. Teacher peer mentoring can be an effective and efficient component of professional development initiatives, but little is understood about ways to support and sustain the mentors themselves. This study employed qualitative methods to explore these issues over two summers of a professional development institute during which 10 teachers experienced in implementing a student-centred instructional model mentored 14 teachers new to student-centred instruction. The teachers and students in the summer institutes came from 13 urban secondary schools serving students underrepresented in STEM who had demonstrated economic needs. Embedded in a larger study of professional development, the current study focused on the experiences and perspective of the mentors as they provided feedback to their mentees. Findings suggested that teacher peer mentoring has the potential to change practice, but that effective mentoring required support and structures for the mentors as they grow into new roles and responsibilities.