Authentic-STEM is a project in which various stakeholders in STEM education and STEM career orientation from schools, universities, businesses, politics, and society work together to create a space for young people to experience self-efficacy in mathematics. This is achieved through long-term problem solving of real problems containing mathematics from businesses as well as contact with their employees moderated by mentors. These mentors are in focus of this contribution. Therefore, after describing the context of the project and problematizing suitable mentoring, a theoretical framework is developed that accounts for the complexity of this problem-solving context. The theoretical framework is then applied on artifacts from different cycles of the Authentic-STEM project. Finally, the paper concludes with an outlook on additional research questions for further development of the mentors’ training and supervision for their and students’ self-efficacy experience.