The purpose of the present study was to investigate if the effects of the parental component of a school-based intervention on dietary fat intake from snacking were mediated by changes in home-related factors. A random sample of 10 schools with 2232 pupils aged 11–15 years was randomly assigned to one of two intervention groups [one with (n = 1226) and one without a parental component (n = 1006)]. Fat intake, home availability of low-fat foods and parental encouragement and support to eat a low-fat diet were assessed with validated self-administered questionnaires. Mediation was assessed with the product-of-coefficient test. Changes in home-related determinants were significantly related to changes in fat intake from snacks; therefore, school-based obesity programmes on adolescents should try to address these determinants. In the present study, one of the three investigated home-related factors, namely parental support, was affected by the parental component intervention. Decreases in parental support were prevented. These changes in parental support were found to mediate the parental intervention effects on changes in fat intake from snacks. Home-related factors appear to be related to changes in adolescents’ snacking behaviours, therefore, school-based obesity programmes should target them. Nevertheless, more research is needed concerning effectively addressing the other determinants.