A co-curricular learning programme, entitled the U-STEMist Scheme, was conducted in a joint university effort to provide STEM undergraduates and pre-service teachers with experience in designing service-based STEM projects. This scheme attempted to link STEM with humanity, which has been undervalued in tertiary STEM education. Students in different majors from four different universities in Hong Kong collaborated to solve real-world problems as a form of community service. From the self-evaluation and feedback of the U-STEMists on the scheme, we found that the scheme was able to develop their basic STEM competencies from setting goals and designing solutions through overcoming various challenges to producing both tangible and intangible products. The U-STEMists valued the experiential learning process, namely, the opportunity to collaborate and learn from their peers with complementary expertise while serving the community. The evaluation outcomes emphasise the benefits of experiential service learning with an emphasis on humanity to tertiary STEM education as a supplement to more narrowly based disciplinary studies. Nevertheless, the outcomes point to the challenges for implementing similar schemes in the future, including strengthening the triadic interrelationship of the U-STEMists, mentors and community partners, better preparation of the U-STEMists for the project and focussing on team building to enhance collaboration among members.