Most studies of school-based mentoring practice have put their key focus on discussions of the professional growth of novice teachers rather than of their mentors. Mentoring practice, however, is also a platform from which mentors can build or enhance their professional competency and capitalize their leadership role as they interact with novice teachers and their colleagues. This is an area which deserves further research attention. Drawing on the concept of social capital, this small-scale qualitative study investigates how mentors can develop or revise their mentoring skills and knowledge while they engage in school-based mentoring practice. Semi-structured interviews and documents were collected from 31 mentors from primary and secondary schools in Hong Kong from 2014 to 2015. Findings of this study firstly show that both bonding social capital and bridging social capital can help mentors develop their mentoring knowledge in different ways. Secondly, bridging social capital can help mentors act as boundary brokers who develop transformative learning by interacting with outside experts. Providing more off-site or cluster-based mentor training programs and mentoring partnership schemes with outside experts could be the way forward to maximize the professional competency of mentors aiming at improving school capacity.