Many South African teachers have low levels of subject knowledge and poor teaching practices but are faced with the additional challenges of implementing a new curriculum and new methodologies for teaching and assessment. These factors combine to expose a teaching population that is generally ineffective, particularly in the case of mathematics and science teachers at under-resourced township schools. The Teacher Mentorship Programme (TMP) is a mentoring programme for mathematics and science teachers focussed on remediating problem areas and revitalising teachers’ classroom practices. Mentors support individual teachers at their workplace and the programme includes all teachers in each of these departments at the schools. This research inquiry is a case study of four TMP teachers. The effect of mentoring as an in-service training strategy is ascertained by considering the changes in their professional, personal and social development. The inquiry reveals firstly the effect of mentoring on the teachers and their practice and secondly what aspects of mentoring are responsible for bringing about such changes. Using the teachers’ and mentors’ voices as informants and based on good practice as recommended by literature, a theory for a mentoring model is proposed as an effective and sustainable model for professional teacher development for mathematics and science teachers in developing contexts. The research findings of this study and the new mentoring model design may serve to enrich the knowledge base on INSET in the area of teachers who are situated in unsupportive schools in developing contexts.