Leadership is an essential component of engineering career success, yet early-career engineers report a lack of leadership skills entering the workplace. Studies have suggested that mentoring opportunities have the potential to provide an alternative approach to learning and practicing leadership. What is not yet understood is to what extent and in what ways serving as a mentor develops leadership. This dissertation fills this knowledge gap by sequentially conducting qualitative and quantitative studies examining how serving as a research mentor influences engineering graduate students and postdoctoral scholars’ leadership understanding and competencies. Study participants were recruited from short-term research programs offered by National Science Foundation (NSF)-funded Engineering Research Centers (ERCs). A total of 17 former ERC mentors and 75 current ERC graduate students and postdoctoral scholars participated in the qualitative study and the quantitative study, respectively. The results suggest that serving as a research mentor can help to advance leadership understanding and competencies. The qualitative study discovered that former ERC mentors believed they gained new perspectives of leadership and developed their leadership competencies while serving as a mentor. This included a growth in awareness of importance to express empathy toward other people and ability to develop others and delivering project results. The quantitative study demonstrated that ERC mentors reported higher competencies in leading other people and delivering project results compared to their peers who had not served as mentors. ERC mentors still primarily connected leadership to leaders, despite the noted gains. This finding indicated the ERC mentors have not yet fully captured the true essence of leadership. The overall evidence suggests that serving as a mentor in a short-term program provided an effective and efficient opportunity for ERC graduate students and postdoctoral scholars to further their understanding of what it means to be a leader and improve their competencies of being a good leader. Such experiences left much to be desired in establishing a social, processual view on leadership.