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.