This paper reports findings from a study of support provided by non-school-based mentors of secondary science teachers in England. It focuses on the identity development of beginning teachers of physics, some of the recipients of the mentoring. Drawing on the analysis of interview and case study data, and utilising third space theory, the authors show how external mentors (experienced, subject specialist teachers who were not based in the same schools as the teachers they were supporting) facilitated opportunities for mentees to negotiate and shape their professional identities, and made valuable contributions to three distinct and important aspects of beginning teachers' identity development. The paper argues that non-judgemental support from external mentors enhances beginner teachers' professional learning and identity development through the creation of a discursive "third" space in which mentees are able to openly discuss professional learning and development needs, discuss alternatives to performative norms and take risks in classrooms. Opportunities for beginner teachers to engage in such activities are often restricted in and by the current climate of schooling and teacher education within England.