Background: The fast-growing trend of urbanizations and the dwellers' stressful lifestyle in megacities has led to several drawbacks from the mental health perspective. Provided that there is a significant association between the green environment and mental health, we investigated different functions of a green roof from two perspectives of well-being and environment. Methods: After investigating on different functions of the green roof and classifying them into two sections, a self questionnaire survey was conducted in May 2020 on 100 citizens of the Seoul metropolitan area after, South Korea visiting the green roof. They were asked to clarify their perception of the different functions of green roof gardens. The statistical analysis was performed to show the differences in perception and how each variable can contribute to such differences. Results: The results illustrated a high satisfaction rate among the visitors of green roofs. The perception of the well-being functions was higher than those of environmental functions. However, age group can alter the perception significantly, as the senior citizens significantly perceived the well-being functions. In contrast, the awareness of the environmental functions was higher among the younger participants. Conclusion: Providing practical information about the ideal functions of green roofs, this study offered helpful insight for the planners who focus on society’s general health and work in two different sections: developing the city landscape and mental health improvement.