Peer-led interventions are highlighted as promising strategies to promote health among adolescents, but little is known about the mechanisms underlying this approach. To better understand the role of peer mentors (PMs) as implementers in school-based health promotion, we combined participant observations, focus group interviews and video recordings to explore high school students' reception of a peer-led intervention component (Young & Active). Young & Active aimed to increase well-being among first-year high school students (∼16 years of age) through the promotion of movement and sense of community and was implemented during the school year 2016-2017 in a larger school-based intervention study, the Healthy High School study in Denmark. The Healthy High School study was designed as a cluster-randomized controlled trial with 15 intervention schools and 15 control schools. At each intervention school, university students in Sports Science and Health (members of the research group) facilitated an innovation workshop aiming at inspiring all first-year students to initiate movement activities at schools. The findings illustrate potentials and challenges implied in the PM role. The peer mentors' profound commitment, as well as their response and sensibility to situational contingencies, were found to be significant for the students' reception and experience of the intervention. In conclusion, the specific job of PMs as implementers seems to consist of simultaneously following a manual and situationally adjusting in an emerging context balancing commitment and identification to the target group and the intervention project.