Purpose: This paper presents the results of empirical research on the effects of teaching practice on student teachers' teaching competence and psycho-emotional development using the "most significant change" narrative investigative technique. Design/methodology/approach: Following a qualitative research approach, the study was conducted during the academic year 2021-2022 in two phases (January and May) with the participation of 73 student teachers of the Department of Theology at the National and Kapodistrian University of Athens (NKUA) during their mentored teaching practice. Student teachers' narratives were collected and analysed using the most significant change technique (Davies and Dart, 2005). Content analysis was conducted with the contribution of seven independent judges. Findings: The results indicate that the Teaching Practice course with the guidance of mentors has a direct and positive effect on student teachers' teaching competence. Mentors also seem to have a positive effect on student teachers' psycho-emotional development. The research also confirms the reliability of the most significant change narrative technique for investigating the effect of related educational interventions. Originality/value: The study empirically validates the usefulness and potential of the investigative narrative most significant change technique for evaluating the effects of teaching practice on student teachers' professional development with the guidance of experienced mentor teachers. The results of the study also have implications for the design and evaluation of teacher practice programmes.