This study explores narratives about critical moments in mathematics learning written by K-8 pre-service teachers’ (PSTs) in the United States over a 20-year period. These critical moments, such as a single memorable task, course, test, or comment from a teacher, had a powerful and sustained impact on PSTs’ mathematics identities, which they carried with them as they entered the teaching profession. We classified narratives using McCulloch et al. (Sch Sci Math 113(8):380–389, 2013) and Drake’s (J Math Teach Educ 9(6):579–608, 2006) categories and found a potentially new category, Taking the Reins. We also classified PSTs’ mathematics orientations as either relational (oriented toward creative problem solving and conceptual understanding) or instrumental (enacting rote procedures without meaning) (Skemp in Math Teach 77(1):20–26, 1976). Many PSTs identified a causal relationship between their mathematics orientations and narrative arcs: relational learning opportunities encouraged positive narratives, while instrumental learning opportunities either resulted in negative narrative arcs or positive but fragile mathematics identities that crumbled under minor stress. We found little variation over time in the nature and prevalence of the narrative arcs and mathematics orientations, suggesting that any changes in mathematics teaching practices over time were not reflected in students’ learning experiences.