CONTEXT Mentors are increasingly involved in doctor performance assessments. Mentoring seems to be a key determinant in achieving the ultimate goal of those assessments, namely, improving doctor performance. Little is known, however, about how mentors perceive and fulfil this role. OBJECTIVE The aim of this paper is to expand understanding of the role of mentors in performance assessment. METHODS Thirty-eight mentors undertook formative performance assessments of their peers in a pilot study. A mixed-methods design was used, consisting of a postal survey (n = 28) and qualitative interviews with a subset of mentors (n = 11). Individual semi-structured interviews were completed and transcripts were analysed by two researchers using a grounded theory approach. RESULTS The results of the survey showed that 89% of mentors intended to continue in their mentorship role. Interviews revealed that mentors used several strategies in the assessments, including: contrasting and collating information; posing reflective questions, and goal setting. Mentors experienced difficulty in disregarding their views of the doctors evaluated. Some mentors noticed obstacles with specific interview skills such as 'paying attention to their colleagues' strengths' and 'enabling doctors to find their own solutions'. Mentors reported that they and their organisations benefited from the assessments. The perceived benefits included: improved interview skills; increased solidarity, and increased mutual respect. CONCLUSIONS The study provides insights into what mentors can do to increase the chance that externally derived information is integrated into doctors' self-assessments. Mainly, mentors used strategies aimed at effectively delivering feedback and encouraging reflection. However, we found that mentors who took part in our study appeared to struggle with a number of obstacles related to: time investment; familiarity with the doctor assessed, and the acquiring of specific interview skills.