In this article we present a case study on a group mentoring practice proven successful in earlier studies in terms of student self-regulation and collaboration. The purpose of our study was to uncover the factors behind the success by interviewing the mentor teachers. The findings showed that the group mentoring focused on four main themes: (a) promoting social relationships, (b) providing personal support, (c) providing study guidance, and (d) strengthening the agency of students. The findings suggest that the leading ethos of group mentoring was related to a humanistic approach to mentoring. However, successful mentoring also seemed to require both situated apprentice and critical constructivist perspectives in addition to the humanistic approach. It is concluded that the combination of multiple goals and contents in group mentoring is the main contributing factor behind the success of the mentoring model examined. Additionally, the teachers reported a variety of positive impacts of group mentoring on teachers’ work.