Peer mentoring is the most common type of peer support framework used in English secondary schools, involving a one-to-one supportive relationship between pupils. Interpersonal benefits have been found for pupils who provide support to others, but there has been little exploration of the experience of being a peer mentor. This qualitative study aimed to provide an initial in-depth look at young people’s experiences. A semi-structured focus group was conducted with seven pupils aged 16 to 17 years in an English secondary school, who were acting as peer mentors to younger mentees or were becoming a mentor. Central thematic categories were identified using abbreviated grounded theory analysis. The central category Becoming grown-ups reflected the way the peer mentors felt the role helped to prepare them for adulthood. Other categories revealed that the role involved a nurturing relationship which benefited mentors but was also challenging. These findings extend the literature by revealing that these pupils may experience personal growth due to both positive and negative aspects of the role. It is recommended that further qualitative exploration of the experiences of peer mentors across multiple settings should be conducted, with findings used to enhance pupil training.