In Ireland state funded support services provide Continuing Professional Development (CPD) for teachers and school leaders. These services are staffed with teachers seconded from their schools on an annual basis for up to a maximum of five years. The policy rationale for this secondment arrangement is rooted in claims that the professional development of these teachers will be enhanced by the secondment experience and that the schools to which they belong will also benefit from this when the teachers return (Department of Education and Skills, 2002, 2006, 2018). There is a significant dearth of studies exploring the professional experiences and learning acquired by teachers while working as teacher educators with Irish CPD support services including how this impacts their future career direction. Additionally, no existing research captures if and how this experience and learning accrues a benefit to the school on their return. Further with an increasing trend in such teachers choosing alternative career routes in the education system post-secondment, there has been no specific research into whether this learning and experience is utilised in other work environments. This study investigates the knowledge, learning and experience acquired by teachers while seconded to Ireland’s largest support service, the Professional Development Service for Teachers (PDST). It also seeks to discover how this influences their postsecondment career. In interpretivist tradition, it employs qualitative interviews to explore this from the perspective of a purposive sample of eight teachers previously seconded to PDST who have since either returned to school or taken up another position in the education system. The researcher’s position as insider is acknowledged given her role as National Director of the PDST. The study’s theoretical framework sets out key stages navigated by these teachers from their initial decision to join the PDST through to their onward post-secondment career destination. Scaffolded by relevant theories and conceptual models in the field of career dynamics, the framework facilitates an exploration of the dichotomies, paradoxes and complexities inherent to the transitions and tensions of this unique career journey. In the context of a considerably under researched area both in Irish and international teacher education, the study’s findings yield fresh insights into the transition from teacher to teacher educator and shifting identities while bringing much needed attention to the distinct role and learning needs of teacher educators working in the CPD sector. In relation to secondment, it exposes the transformational impact of what is intended as a temporary career route and its effects on career aspirations and agency. Findings pertaining to post-secondment shed light on the factors which influence knowledge sharing and construction in schools and in other education work settings. They present Irish secondment policy as a significant issue in its lack of provision for these realities while its implications for continuity and preservation of expertise in the PDST emerge as deeply problematic. Study recommendations centre on career pathways for teachers seconded to continuing teacher education and a review of current secondment stipulations. The recommendations are situated within the broader context of the professionalisation of continuing teacher educators and a need for a greater understanding of knowledge sharing and capacity building in the system.