There is a burgeoning interest in the concept of identity within mathematics education research, with recent work suggesting that the interplay of identity and capital (cultural, social, and economic resources) offers a productive lens for understanding school students' trajectories into, or away from, mathematics. This paper adds to understandings of how interactions of identity and capital play out over time, focusing on how these mediated the mathematics degree trajectories of three young men, Tom, Neb, and Gerrard, who were selected from a wider longitudinal study, as the only young people who went on to take mathematics degrees. Analysis of longitudinal data from 24 semi-structured interviews, conducted with the three young men from age 10 to 21 and two of their parents/carers, found that all three identified as being "good/exceptional at mathematics". These identifications developed at an early age, were sustained and augmented by capital, and were closely related to pursuing the subject at university and enjoyment of mathematics. However, developing and sustaining an identity as "good at maths" relied on interactions with mathematics capital through families, school, and wider networks. Moreover, classed differences in the distribution of capital were implicated in their different degree outcomes. We argue that attending to the longitudinal interplay between capital and identity offers a rich understanding of how young people come to see themselves and be seen by others as "naturally able" at mathematics and in turn supports choosing to study a mathematics degree. Implications for mathematics education policy and practice are considered.