Mathematics teacher educators in teacher education for primary school need expert knowledge and skills in teaching in primary school, in subject matter, and in research. Most starting mathematics teacher educators only partly possess this knowledge and skills. We developed and tested a two-year professional development trajectory for starting primary school mathematics teacher educators using design-based research to evaluate and improve the design. In this paper we describe the background and the design of the professional development trajectory and the experiences with several cohorts of starting mathematics teacher educators. We conclude that the professional development design’s focus should be on mathematical knowledge for teaching, should refer to both teacher education and primary education practice, should offer opportunities for cooperative learning, and has to use practice based research as a developmental tool.