Mental health is a major issue worldwide and people—whether or not they have personal experience with mental health problems—can benefit from mental health education. The aim of this study was to explore adolescents’ perceptions of mental health and to ascertain whether a mental health educational intervention can alter their perceptions. Individual interviews with open-ended questions were conducted and data were subjected to content analysis. A sample of 59 participants (between 13 and 16 years of age) from two Greek secondary schools agreed to participate. The findings demonstrate that adolescents can provide a clear description of mental health and what it entails following the intervention and that their understanding improved. They also acquired a more sophisticated way of thinking about mental health. The findings are encouraging for future research in this area, and highlight the importance of adolescent mental health promotion and the ways it can be implemented in practice.