This longitudinal study examined academic self-efficacy and performance among Science/Technology/Engineering/Math (STEM) majors who are underrepresented in STEM education and occupations; i.e., women, specific ethnic minorities, and low-socioeconomic status (SES) individuals. Performance on academic tests and self-perceptions of academic skills were assessed at admission and graduation from a STEM mentoring program. At admission, women perceived themselves as academically weaker than men despite similar academic performance. However, by graduation, women's academic self-efficacy was equivalent to men's. In addition, students with double STEM-minority statuses, by ethnicity and SES, had lower academic self-efficacy and performance than d id students with single STEM-minority status. Exploratory analyses of change over time by ethnic/SES groups showed varying patterns of change that depended on the outcome variable. This study's finding of an increase in academic self-efficacy for women and students with STEM-minority status by both ethnicity and SES at graduation from a mentoring program is perhaps an indication of the positive impact of mentoring. The mixed findings at program completion for students with single versus double STEM-minority status call for attention to the complex relationship between social disadvantage, academic self-efficacy, and academic performance.