Gender-based violence is a global public health issue and major human rights concern. It is also a type of violence that is disproportionately experienced by women and girls. This study is the first to examine multiple implementation process (dosage, fidelity, and adaptation) effects on changes in anticipated outcomes of a school-based bystander program targeting gender-based violence, Mentors in Violence Prevention (MVP). Data were collected from two participant groups: mentees (students receiving MVP) and mentors (students delivering MVP), across nine participating high schools. The mentee sample comprised 698 students (about 48.9% males and 49.7% females), aged 11 to 14 years old ( M = 11.86, SD = 0.64). The mentor sample comprised 118 students (17.80% males, 82.20% females), aged 15 to 18 years old ( M = 16.42, SD = 0.60). Anticipated outcomes were changes in bystanders’ attitudes, social influences, control perceptions, intentions, willingness, and intervention behavior, measured using mentees’ self-reports at two time points approximately 1 year apart. Implementation processes were measured using mentors’ self-reports. Analyses revealed no effects for any of the implementation variables across changes in any of the outcomes measured. These results highlight important implications for the implementation of the MVP program going forward, given its widespread implementation in the United Kingdom. Possible ways that MVP may be enhanced in future are discussed. For example, furthering understanding into how gender-based violence and bystander intervention are addressed and framed during MVP lessons would give more insight into how the current implementation of the program can be improved to maximize its potential benefits.