Engaging in argumentation from evidence is challenging for most middle school students. We report the design of a media-based mentoring system to support middle school students in engaging in argumentation in the context of a game-infused science curriculum. Our design emphasizes learners apprenticing with college student mentors around the socio-scientific inquiry of a designed video game. We report the results of a mixed-methods study examining the use of this media-based mentoring system with students ages 11 through 14. We observed that the discourse of groups of students that engaged with the game-infused science curriculum while interacting with college student mentors via a social media platform demonstrated statistically significant higher ratings of cognitive, epistemic, and social aspects of argumentation than groups of students that engaged with the social media platform and game-infused science curriculum without mentors. We further explored the differences between the Discourses of the mentored and non-mentored groups. This analysis showed that students in the mentored groups were invited, guided, and socialized into roles of greater agency than students in the non-mentored groups. This increased agency might explain why mentored groups demonstrated higher levels of scientific argumentation than non-mentored groups. Based on our analyses, we argue that media-based mentoring may be designed around a video game to support middle school students in engaging in argumentation from evidence.