There is a need to address disparities in the underrepresented minority (URM) STEM students' degree attainment and retention, vs. non-URM students. A literature review suggests that URM STEM students face barriers related to demographic, academic, and social-cognitive factors. The Louis Stokes North Star STEM Alliance (LS-NSSA) seeks to address these factors and promote URM student success in STEM disciplines. However, there is a gap in knowledge regarding LS-NSSA's mechanism and outcomes. The current study investigates factors influencing URM STEM students' retention and graduation, investigates LS-NSSA program effectiveness, and investigates the path by which outcomes are achieved. A theory-based evaluation approach is utilized in an explanatory sequential mixed-methods design guided by LS-NSSA's theory of change. The results of three sub-studies suggest the importance of first-semester experience and academic outcomes for URM STEM students' graduation and retention. Participation in LS-NSSA is associated with a higher level of academic preparation to pursue a bachelor's degree, compared to URM students not affiliated with the program. URM STEM students participating in LS-NSSA's research mentorship programs have higher levels of confidence, interest, science identity, sense of belonging, and commitment to STEM. The current study offers evidence supporting LS-NSSA's theory of change regarding student academic and social-cognitive trajectories. The current study suggests directions for future evaluative studies of LS-NSSA. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]