Recent research suggests that mentors can enhance their mentees' mental health. Yet little is known about the effect of mentoring on the anxieties of the mentors themselves. We hypothesise that mentoring programmes can have a positive impact on both mentees' and mentors' mental health. In a multi-method longitudinal study, we explore empirically the effect of a formal mentoring programme in the English police force on senior police officers' levels of anxiety. In the first step of our study, we conducted a field experiment. This experiment yielded preliminary results that suggest mentors may experience marginally lower levels of anxiety and feel as if their job is more meaningful when they participate in the mentoring programme. As a second step, we qualitatively investigated the subjective experience of the participants to explain and substantiate our findings. We found that mentoring relationships provide a unique context for mentors to discuss and normalise their concerns, to share ideas for managing anxieties, and to find more meaning in their work. We discuss our findings in terms of mentoring theory and consider their practical implications for improving mental health in the workplace.