The purpose of this study was to investigate how the benefits described by undergraduate mentors in an afterschool STEM program for elementary students changed over time the longer they served as mentors in the program. We used the afterschool program as a case study and drew on interviews with 30 mentors who served between one and six semesters in the program between 2017 and 2022. We found that the mentors developed their competence, enhanced their autonomy, and built new relationships. How and the extent to which the mentors experienced these benefits shifted over time the longer they served. Our findings add to existing research on how mentors' experiences change over time and have implications for mentoring programs and how they support and retain mentors.