Background: For Inspiration and Recognition of Science and Technology (FIRST) robotics is an international, extra-curricular program that fosters young students' interpersonal skills and career choices in science, technology, engineering, and mathematics (STEM). FIRST teams are guided by mentors, about half of whom are also mentees. Purpose: To describe and characterize FIRST mentors and their perceptions of their own interpersonal skills and STEM career choice and identify differences by mentor types and gender. Method: The study participants included 261 FIRST mentors. A convergent mixed-methods approach was used. Data was collected quantitatively via questionnaires and qualitatively via interviews. The analysis was guided by the social cognitive career theory (SCCT). Results: Fourteen categories were identified to describe and characterize the FIRST mentors. Nine were based on SCCT and five were new: influence of friends, interpersonal skill, personal contribution, challenges, and mentor-as-educator. Differences were found between three types of FIRST mentors: non-FIRST mentors, graduate mentors, and mentee mentors. The correlations between factors and categories indicated that the mentors' perceptions were consistent with those of the mentees. The FIRST program impacted the STEM career choice of men more than women, and external motivation influenced women more than men. Conclusions: The FIRST program contributes to developing its graduates' interpersonal skills and affects their STEM career choice. The study provides insights into mentors' influence on mentees' career choices, from which both genders benefit. Making FIRST available in schools as a widespread enrichment program is expected to foster students' STEM career choices, thereby contributing to the human resource reservoir of the high-tech industry workforce.