The aim of this article is to introduce and reflect on the development of mentoring in Finland as implemented in the form of the Peer-Group Mentoring (PGM) model. Firstly, the main characteristics and principles of the PGM model are introduced, after which experiences from implementing the model are analysed based on a literature review. We examine the following research questions: (1) How have the mentoring practices of newly qualified teachers developed towards the PGM model within the educational ecosystem in Finland? (2) How has the PGM model found its relational space (‘ecological niche’) in this ecosystem? and (3) How has PGM been experienced by mentors and mentees in terms of professional learning and well-being? Theoretically, our examination is based on an ecosystemic view: we see mentoring as an integral element of teachers’ professional development, which, in turn, is an integral part of the education ecosystem. Our methodological approach is a systematic review of mentoring research in Finland over the period 2004–2019. The research material consists of policy documents, research reports, peer-reviewed articles and doctoral dissertations. Our research: (1) demonstrates how mentoring has evolved gradually from traditional one-to-one mentoring towards a reciprocal, collegial and dialogic approach; (2) shows how the PGM model has inhabited its niche in the ecosystem of education; and (3) presents how PGM has been experienced by mentors and mentees. The main challenge in applying the model is the lack of national agreement concerning the organisation of mentoring. Applying the concept of learning ecology, mentoring can be seen as struggling for a niche within initial teacher education and in-service training. Finding a sustainable solution for mentoring therefore requires developing the ecosystem as a whole and allowing growing space for mentoring as an integral part of the education ecosystem.