School-based mentoring (SBM) facilitates nurturing relationships between adolescent student mentees and older and more experienced mentors, often to address concerns regarding aspects of the mentees’ social-emotional development. Though there is a body of evidence that demonstrates the effectiveness of SBM in supporting mentees towards resolving these developmental concerns, our understanding of how and why it is effective is continuing to develop. The role of the mentor in effecting positive developmental outcomes, particularly in a professional capacity, is currently not well understood. This study investigated the lived experience of professional mentors and the ways in which this lived experience shaped their conception and operationalisation of the role. Participant diaries, semi-structured interviews and follow-up interviews were employed as data collection methods in a hermeneutic phenomenological research design. The data analysis conducted for this study also employed a hermeneutic phenomenological method that developed a descriptive and interpretative account of the lived experience of professional mentorship. Findings from this study suggested that the conceptualisation and operationalisation of professional mentorship is shaped by the relational and organisational imperatives operating within formal SBM programs. The elements and dimensions of these imperatives were seen to directly influence the lived experience of professional mentorship. While the findings of this small-scale study must be viewed tentatively, they will be of interest to mentors, youth workers, success coaches, social workers, teachers and the current literature on youth mentoring.