High-quality mentoring relationships help college students to achieve their academic potential and career goals. However, less is known about factors that shape the antecedents of mentoring relationships (e.g. mentor-protégé psychological similarity), or the impact of similarities on the development of high-quality mentoring relationships. The current study advances mentoring theory by testing effects of research apprenticeship program involvement on perceptions of psychological similarity and the development of high-quality mentoring relationships over time. Undergraduate students (N = 170) in a research apprenticeship program and matched control students completed survey measures of mentoring experiences at the end of fall (Time 1) and spring (Time 2) semesters. Longitudinal structural equation model results show that involvement in the program promoted similarity, which in turn promoted support and relationship quality over time. Furthermore, early development of relationship quality strengthened the future experiences of psychosocial support from and role modeling identification with the faculty mentor.