In this study, a qualitative, longitudinal research design was used to characterise participants’ mathematical identity, and investigate how this changed during their transition to university mathematics education. The study extends previous research on mathematical identity in Ireland to include a previously under-researched cohort of science and engineering students. A definition of mathematical identity was operationalised under a narrative paradigm, meaning that identity was seen as fluid and ever-changing, and that narratives were positioned as “enactments of identity, constructed in the moment” (Radovic et al., 2018, p. 29). The study included three sequential stages of data collection, which combined reflexive thematic analysis of questionnaire (n = 32) and focus group (n = 5) data with an in-depth narrative analysis of individual interviews (n = 6). This research found further evidence that reflecting on their own mathematical identity helps students to engage more effectively as mathematics learners (Kaasila, 2007b) and process new mathematical experiences (Sfard & Prusak, 2005, p. 16). The results of this study are of interest to practitioners who seek to improve the overall learning experience of science and engineering students, and the efficacy of the teaching they encounter. The narrative methodology and methods used are of interest to a wider audience of researchers in affect and identity. The key findings include that participants presented only absolutist views of the discipline of mathematics but saw the value in relational understanding and problem-solving when it comes to real-world applications. They also believed that progression through mathematics at Senior Cycle and university is based on hard work, interest, and passion, not natural ability. Over time, participants collaborated more with their classmates, and relied on sources of learning outside the classroom as well as inside. Teaching and work placements, in which participants learn the role of mathematics in their intended work environments, were found to have a significant influence on participants’ mathematical identity.