Numerous national and state endeavors have advocated for approaches, funding, and programs focused on expanding the science, technology, engineering, and mathematics (STEM) workforce of the nation and investing in education to cultivate a more diverse and impactful cohort of students who pursue STEM pathways. Educators, support personnel, and policymakers are in a position to engage in discussions about expanding STEM college readiness (STEM-CR) and participation. However, few are cognizant that STEM-CR is a progression that students strive for in developing skills, behaviors, and attitudes that spans over time. The current study established and validated a measurement model of student STEM-CR in mathematics and science utilizing the High School Longitudinal Study of 2009. The model was created based on a multidimensional and theoretical perspective of college readiness using a confirmatory factor analysis and modeling approach that accounted for measurement invariance. The sample (N = 16,044) comes from a racialized/ethnoracial and socioeconomically diverse high school population in the United States. The findings confirmed that STEM-CR involves four related yet distinct dimensions of Think, Know, Act, and Go. Results also demonstrated soundness of these STEM-CR dimensions by race and gender (key learning skills and techniques/Act). Academic self-efficacy was the strongest dimension of our STEM-CR model and strongly predicted academic achievement and college enrollment. Research and practice implications are discussed.