This study applies Eccles’ expectancy-value theory to explore the influence of ninth- grade STEM motivational beliefs on students’ subsequent academic trajectories in high school and college. Specifically, it investigates how these beliefs affect high school STEM course choices, STEM achievement, graduation rates, and college STEM major enrollment, focusing on two key subgroups: gender and English Language Learner (ELL) status. Given the underrepresentation of female students in STEM fields and the academic performance challenges faced by ELL students, this study provides insights on STEM motivational dynamics and educational outcomes aimed at enhancing their representation in STEM. Utilizing the nationally representative High School Longitudinal Study of 2009, the findings reveal that strong STEM motivational beliefs in ninth grade are positively associated with higher STEM achievement, more STEM courses taken, and a greater likelihood of enrolling in STEM majors at the college level. The results indicate that while female students exhibit lower STEM motivation compared to their male counterparts, they generally perform better in STEM courses. Nonetheless, they are less likely to choose STEM majors in college. The findings also reveal that improving female students' STEM motivation and STEM GPA could proportionately increase their enrollment in STEM fields. In contrast, no significant differences in STEM motivation were observed between ELL and non-ELL students. However, ELL students generally show lower STEM performance and graduation rates. Among those who graduate from high school and enroll in four-year colleges, ELL students are just as likely as their non-ELL peers to choose STEM majors. These insights provide guidance for policy makers on improving female students' STEM motivation and supporting ELL students in high school academic achievements to address the gaps in STEM field.