The biophilic city concept has been proposed as an approach to mitigate the negative impacts of urban growth under global warming, which increasingly affects people’s mental health. As architecture occupies a substantial proportion of urban areas, biophilic architecture plays a crucial role in supporting urban well-being, contributing to sustainable development in line with Sustainable Development Goal 3 (SDG 3) (Good Health and Well-being). Previous studies have identified a research gap regarding the integration of exterior biophilic architectural attributes within urban contexts. Therefore, this study aims to identify exterior biophilic architectural attributes that can enhance urban mental well-being, using buildings in Singapore as case studies. A questionnaire survey was conducted to evaluate architectural attributes and people’s emotional responses, with the aim of analyzing their relationships. The results indicate that exterior biophilic architectural attributes contribute unequally to urban mental well-being. Natural features emerged as the most influential attribute, exerting comprehensive positive effects on attentiveness, inspiration, and self-assurance. Natural forms and natural colors also demonstrated significant contributions by promoting relaxation, cognitive engagement, and inspiration. In addition, natural spaces supported attentiveness and confidence, while natural materials, despite being less prevalent, exhibited strong qualitative effects on inspiration and psychological security. Accordingly, this study provides biophilic architectural design guidelines that support the mental sustainability of urban populations affected by global warming.