A constant influx of newcomers is essential for the sustainability and success of open source software (OSS) projects. However, successful onboarding is always challenging because newcomers face various initial contributing barriers. To support newcomer onboarding, OSS communities widely adopt the mentoring approach. Despite its significance, previous mentoring studies tend to focus on the newcomer’s perspective, leaving the mentor’s perspective relatively under-studied. To better support mentoring, we study the popular Google Summer of Code (GSoC). It is a well-established global program that offers stipends and mentors to students aiming to bring more student developers into OSS development. We combine online data analysis, an email survey, and semi-structured interviews with the GSoC mentors to understand their motivations, challenges, strategies, and gains. We propose a taxonomy of GSoC mentors’ engagement with four themes, ten categories, 34 sub-categories, and 118 codes, as well as the mentors’ attitudes toward the codes. In particular, we find that mentors participating in GSoC are primarily intrinsically motivated, and some new motivators emerge adapting to the contemporary challenges, e.g., sustainability and advertisement of projects. Forty-one challenges and 52 strategies associated with the program timeline are identified, most of which are first time revealed. Although almost all the challenges are agreed upon by specific mentors, some mentors believe that several challenges are reasonable and even have a positive effect. For example, the cognitive differences between mentors and mentees can stimulate new perspectives. Most of the mentors agreed that they had adopted these strategies during the mentoring process, but a few strategies recommended by the GSoC administration were not agreed upon. Self-satisfaction, different skills, and peer recognition are the main gains of mentors to participate in GSoC. Eventually, we discuss practical implications for mentors, students, OSS communities, GSoC programs, and researchers.