Teachers' professional learning is a desired goal, which contributes to the improvement of educational outcomes. Some claim it can take place on the job, specifically when teachers co-teach with and mentor student-teachers. Nevertheless, only little is known regarding student-teachers' role in shaping the learning experience of mentor-teachers. We present findings from a multiple case study focusing on mentors' professional learning. We explain how student-teachers' professional agency and use of resources may create professional learning opportunities for mentors and influence their learning. We contend that unpacking this process can provide information on how teacher preparation programmes can cultivate student-teachers' agency and their influence upon mentor-teachers' learning.