Background: A strong calculus foundation is essential to undergraduate engineering success. However, some students may be self-selecting to begin their mathematics sequence in a lower-level calculus course than their prior achievement and aptitude would suggest is appropriate (i.e., undermatch). Purpose: This study examined (a) the relationship between engineering students' academic outcomes and first-year calculus course taken, (b) the extent to which first-year engineering students select calculus courses appropriate for their prior mathematics achievement and background, and (c) students' rationales for calculus course selection. Design/Method: The study used a mixed-methods approach consisting of quantitative "t"-test, multiple regression, and classification decision tree model analyses of student records of (a) first-year engineering students from 2009 to 2016 (n = 2689) at a highly selective public research university and (b) qualitative focus groups of 95 undergraduate engineering students in 2017-2018. Results: Students who begin their math sequence in Calculus I had lower graduating grade point averages, longer time-to-degree, and were less likely to major in popular engineering fields than those who started in Calculus II. Of first-year undergraduates, 18.4% undermatched their choice of calculus course. Students' rationales for choosing a particular calculus course included (a) prior achievement, (b) recommendations from others, and (c) self-confidence. Conclusions: Results suggest that undermatching the choice of first calculus course may lead to negative consequences for students' STEM pathways even at a highly selective engineering school. These results hold implications for practice, especially in terms of advising at the high school and college levels.