Background: Mental disorders most likely manifest during adolescence, providing opportunities for schools and professionals to implement standardized mental health literacy (MHL) interventions to support improvement of mental health outcomes. Purpose: This study aimed to evaluate the impact of an MHL intervention (the Guide) on knowledge, stigma, help-seeking and overall mental health among grade 8-9 students. Methods: We developed a longitudinal cohort study to evaluate the Guide among 523 students, delivered across subsequent grade levels within a Catholic school board in Canada. We utilized validated measures to assess mental health knowledge, stigma, help-seeking attitudes, and mental health across four time-points: baseline, end of grade 8, beginning of grade 9, and end of grade 9. Teachers undertook training to learn how to administer the intervention, utilize included teaching aids, and collect data. Results: Students demonstrated improvements on knowledge and stigma at all measurement intervals, however statistically significant differences in help-seeking and well-being outcomes were not observed. Discussion: This study suggests the plausibility of longer-term delivery of the Guide and similar resources in enhancing knowledge and reducing stigma among similar youth cohorts. Translation to Health Education Practice: Certified Health Education Specialists may adapt the Guide to accommodate youth mental health needs in different contexts.