This study explores the link between a mentorship program for in-service teachers and their professional development. The participants are under-qualified mathematics, science, and technology teachers employed in junior secondary schools. Using a mixed-method approach, data were collected through a questionnaire responded to by 163 mentees, and semi-structured interviews with 4 in-service teachers. Despite tensions between mentors and mentees, the results show a strong link between mentoring and the professional development of in-service teachers. The majority of the sampled teachers learnt new teaching approaches and improved their content knowledge. The findings are emergent, suggesting that experienced in-service teachers require different models of mentoring relationships between mentors and mentees to those suggested in the literature.