Mentoring is intended to guide student teachers towards pedagogical decision-making within the complexity of the classroom. In this article we focus on the mathematics mentoring practices of 1 primary school teacher at a university-affiliated teaching school in Johannesburg. The teacher had many years of experience and had undergone some development as a mentor over a period of 5 years. With the research reported on here, we aimed at gaining insight into how the teacher conceptualised her mentoring encounters with student teachers and how this aligned with her mentoring practices and processes. Using a qualitative case study design and multiple methods of data collection, the findings point to the mentor teacher making substantial progress towards functioning as an expert teacher. However, the data confirmed a fluctuation in enactment of mentoring between experienced and expert teacher, in both generalised and subject-specific contexts. The implications of the findings are relevant for the thousands of school teachers who mentor student teachers and novice teachers, particularly in primary schools where teachers are often generalists and not subject specialists. The findings could also inform guidelines and policy briefs for training by higher education institutions, the Department of Basic Education (DBE) and the South African Council of Educators in order to support the development of expert mentor teachers.