Background: Twenty-first-century professional skills are vital to success in science careers and can be developed through participation in course-based undergraduate research experiences (CUREs). However, CURE research rarely explores outcomes of undergraduates who serve as mentors and their perceptions of their skill development. Purpose: This study investigated how undergraduate biology students perceived their development of 21st-century professional skills as a result of Inclusive Biologists Engaging in Active Research with Students (iBEARS), a CURE approach to experiential learning that positioned them to serve as research mentors to local classes of K–12 students. Method: Using a phenomenological case study design, we analyzed pre- and post-CURE reflection data from 24 of the undergraduates and follow-up interview data from four former CURE participants to examine their perceptions of developing the following competencies: communication, collaboration, critical thinking, and creativity. Findings: Thematic analysis revealed that participants recognized their utilization of skills during CURE participation but also felt that they had strengthened each skill. Communication skills were the most recognized and discussed by participants, while creativity skills were the least discussed. Implications: Educators should work to broaden access to CURE curricula that offer opportunities for undergraduates to serve as mentors to younger students. Future research should include analyses of CURE components and their resulting development. Significance of Research Statement: This study shows how CUREs and serving as research mentors can develop college students’ professional skills in science.