Although implementation fidelity (IF) is an important factor in interpreting the outcomes of intervention programs, so far there is little knowledge about how it actually relates to them. The purpose of this systematic review was to identify which component of IF (adherence, quality of the intervention, exposure to the intervention and receptiveness) is the most important one for attaining the expected results in school-based mental health programs. A search in four electronic databases (APA PsycNET, PUBMED, EBSCO, and ISI-WEB Science) yielded 31 articles published between 2006 and 2016 that met all the established inclusion criteria. To determine the associations present, the proportion was calculated between the number of times that the components of IF were significantly linked to outcomes and the total number of times that the association was evaluated. It was observed that the various components of IF are linked to outcomes 40% of the time and that the strongest association is established with students' exposure and receptiveness to the intervention. Lastly, findings and their implications for future research are discussed.