The development of professional digital competencies is an important condition for a meaningful integration of digital technologies in mathematics education. A widely used and empirically studied approach for the professional development of in-service teachers are teacher design teams, in which lessons are collaboratively planned, implemented and reflected. However, this approach has not yet been considered for the collaboration of pre-service and in-service teachers. Therefore, this article examines in a holistic case study mixed teacher design teams composed of pre-service and in-service teachers. A total of 23 interviews were conducted with pre-service and in-service teachers and analyzed by inductive development of categories using the method of qualitative content analysis. The case study shows among other things, that the different prerequisites of the participants can lead to a relatively clear division of roles within the collaboration, which turns out to be suitable for the professionalization of both pre-service and in-service mathematics teachers.