Enjoyment of physical activity (PA) is positively correlatedwith PAengagement. The inclusion of peers has been found to increase the likelihood of PAenjoyment in youth. Peer-led strategies, incorporating peer networks in the intervention delivery, is relatively underused and consequently understudied in school-based PA interventions. The purpose of this investigation was to evaluate the feasibility and acceptability of the novel Girls Peer Activity (G-PACT) peer-ledmentoring intervention. Two-hundred and forty-nine Year 9 adolescent girls (13–14 years old) fromthreemixed-sex secondary schools located inWest Lancashire, North-West England were invited to participate in the G-PACT project. The study employed a novel approach by using a three-tier model, including (Tier 1) Mentors (undergraduate students), (Tier 2) Leaders (Year 9 girls selected by teachers), and (Tier 3) Peers (whole Year 9 cohort). Mentors delivered a series of educational and leadership training to the Leaders in each respective school who then disseminated this information to their Peers and encouraged themto engage inmore physical activities. Eight focus groups were conducted with Leaders (n = 40), 28 focus groups with Peers (n = 185), two focus groups with Mentors (n = 6), and three interviews with teachers (n = 4). Thematic analysis was used to analyze the pooled data and identify the key themes. The study found that the G-PACT intervention was feasible and acceptable for adolescent PA Leaders and theirMentors. The relationship between Leaders and their Peers required refinement to improve the communication processes to increase Peer engagement in the G-PACT project.