This qualitative study explored 8 urban mentor teachers' experiences and perceptions of mentoring in a yearlong field placement in an Urban Teacher Residency program. Results indicate mentors' and preservice teachers' joint work provided a context conducive for professional learning and contributing to a larger social justice mission. However, ongoing collaboration also produced tensions, some of which mentors had difficulty navigating. This study suggests that if extended field placements are to be an effective reform intervention, mentor teachers need professional support to maximize the benefits and to negotiate the complexities of collaboration.