This research investigated the role of mathematics in engineering practice and whether there is a relationship between students’ experiences with school mathematics and their choice of engineering as a career. The study was inspired by the observation that there is a lacuna in the scholarly literature concerning the nature of mathematics’ role, if any, as a significant cause of the declining number of students entering professional engineering courses. Additionally there is currently no broad picture of the mathematical expertise required or used by practising engineers. The population of interest in this study comprises professional engineers practising in Ireland. A sequential explanatory mixed methods design, where the subsequent collection and analysis of interview data builds on the survey findings, is employed. Engineers’ use of mathematics is considered in three parts: curriculum mathematics, mathematical thinking, and engaging with mathematics. Curriculum mathematics usage is measured by a derivation of de Lange’s mathematics assessment pyramid and with reference to three dimensions: mathematics domain, usage type, and academic level. Thinking usage relates to mathematical modes of thinking. Engaging usage is the motivation to take a mathematical approach. Engineers’ experiences of school mathematics, factors that contributed to their engagement with school mathematics and the impact of their feelings about mathematics on their choice of engineering careers are investigated. The findings show that (i) engineers’ feelings about mathematics are a major influence on their choice of engineering as a career; (ii) teachers, affective factors and sociocultural influences are the main contributors to engineers’ interest in and learning of mathematics; (iii) while almost two thirds of engineers use high level curriculum mathematics in engineering practice, mathematical thinking has a greater relevance to engineers’ work compared to curriculum mathematics; (iv) professional engineers’ curriculum mathematics usage is dependent on the interaction of engineering discipline and role and their mathematical thinking usage is independent of discipline and role; (v) engineers show high affective engagement with mathematics and their usage of mathematics in engineering practice is influenced by the value given to mathematics within their organisation; and (vi) the focus on “objective” solutions at the expense of tacit knowledge in mathematics education reduces the value of mathematics in engineering practice.