The most common approach to foster graduate attributes is to teach them in the curriculum of a bachelor’s degree. However, it is difficult to include every graduate attribute in every degree. In this article we consider how co-curricular peer mentoring might provide an additional approach. We examine a case study of the mentors of the Peer Assisted Study Sessions (PASS) programme at a research-intensive university in New Zealand, and we examine the process by which they developed graduate attributes. PASS mentors reported that they developed a range of graduate attributes such as communication, critical thinking, and ethical responsibility, due to the extra responsibility and leadership involved in being a mentor in an authentic work environment. We argue that co-curricular programmes such as PASS can provide useful additional opportunities for students to acquire and develop graduate attributes. While not all students will be able to participate as PASS mentors, we also argue that our findings can inform other programmes for fostering graduate attributes. If these programmes offer authentic responsibilities to participating students, they may be more effective at fostering graduate attributes.