This full research category paper examines how first-generation college students' mathematics and physics identity support the development of an engineering identity and how the three STEM identities support students' commitment to choose an engineering major. This study focuses on first-year, first-generation college students in engineering (n=616). The purpose of only focusing on first-year students was to understand how their decision to pursue engineering is shaped before engaging in engineering or college coursework. Structural equation modeling was used to test direct and indirect paths of students' STEM identities on engineering major commitment. First-generation college students' perceptions of themselves as capable and interested mathematics learners supported their views of themselves as capable physics and engineering learners. Forming an identity as a math and physics person supported the development of an engineering identity. First-generation college students' engineering identity supported their confidence of choosing an engineering major. However, female and Latinx first-generation college students were less likely to see themselves as physics people. Female first-generation college students were also less confident about their choice to pursue engineering. The findings elucidate the difference each subject-related role identities have on first-generation college students' developing engineering identity and confidence in their major choice.