The misassignment of teachers, with respect to their qualification, results in teachers teaching mathematics out-of-field (OOF). OOF mathematics teaching is an international area of concern, with research documenting the existence of the practice in Australia, the United States, Germany and England, and the negative effects that the practice can have on teachers’ lives, student learning and the professional standards of teaching. Research conducted in Ireland has demonstrated the prevalence of teachers being assigned to teach mathematics OOF in post-primary schools. In response to the issue of OOF mathematics teaching, the Irish government funded the Professional Diploma in Mathematics for Teaching (PDMT), a professional development programme for teachers teaching mathematics OOF, enabling them to obtain mathematics teacher certification. The content of the PDMT, then, adheres to the Teaching Council of Ireland’s curricular requirements for teaching mathematics. This research employs the construct of first-person mathematics-related (1st PM-R) teacher identity to capture the learning trajectory of a participant on the PDMT. 1st PM-R teacher identity refers to the teacher’s self-understandings in relation to mathematics teaching. These self-understandings are stories told by the teacher about themselves to a third party. This paper reports on the 1st PM-R teacher identity of Mary, a certified business studies and accounting teacher teaching mathematics OOF, whilst undertaking the PDMT. Thematic analysis and sociolinguistic tools were used to analyse interview data to obtain insights regarding Mary’s 1st PM-R teacher identity, and the ways in which her 1st PM-R teacher identity was reshaped over her involvement in the PDMT. The findings suggest that professional development programmes have the capacity to impact 1st PM-R teacher identity, in particular, with regard to how the teacher believes they are seen by school management and their students’ parents; however, other factors, such as colleagues and previous experience learning mathematics, may be more significant in shaping the teacher’s 1st PM-R teacher identity in relation to their teaching practices. The findings from this research may have implications for the development of mathematics professional development programmes and the criteria for determining mathematics teacher certification.