Purpose The purpose of this paper is to explore beginning teachers’ perceptions of the role of the mentor in the early stages of developing a professional identity. The beginning teachers in the authors’ study are defined as having been awarded qualified teacher status at the end of an initial teacher education programme or having completed their first term as a new teacher with responsibility for a class of pupils. Design/methodology/approach The research design was a qualitative, inductive study. The concepts of communities of practice, legitimate peripheral participation and power dynamics within a community underpinned this study. The data set was collected over a period of 18 months, through six focus groups and 40 questionnaires with beginning teachers across 34 schools altogether. The data set was analysed using interpretive phenomenological analysis (IPA). Findings The findings indicated that the ways in which mentors use their power to recognise (or not) the legitimacy of beginning teachers as being part of the school community influences the development of beginning teachers’ professional identities. The thematic analysis of the data indicated the different types of support that mentors may provide: “belonging”, “emotional”, “pedagogical” and “space”. Research limitations/implications Further research into how mentors perceive their role in supporting new entrants into the profession is needed. Originality/value These findings are pertinent in England, as the increase in school-based initial teacher training provision will intensify the role of school mentors. These findings will be of value to other countries that are moving towards an increase in school-based teacher training.