Teacher leadership is increasingly being touted as a practical response to guide teacher learning in school improvement and policy reform efforts. However, the field of research on teacher leadership in relation to post-compulsory educational development has been and remains largely atheoretical to date. This empirical study proposes a grounded theory of teacher leadership development based on the lived experiences of self-selected teacher leaders in the USA, that have been explored and validated through further data collection with teacher leaders from England, Scotland, Wales, and the Republic of Ireland. The theory posits leadership development as an iterative and recursive, rather than linear, experience that centres on the construct of personal growth, but also includes the overlapping constructs of growth as a teacher, researcher and leader. Such growth is underpinned and guided by significant teacher development through various post-compulsory developmental activities. Teacher leadership development is situated within the context of the classroom, school and community. The interactions between teacher and context create feedback loops that either facilitate the achievement of specific impacts or challenge the teacher's further development. This study contributes a visual model of teacher leadership development to the field.