Drawing on the sociocultural theoretical (SCT) perspective, we offer a microgenetic analysis of mentor-mentee interactions in the context of pre-service teachers' practicum in a Master's program for Teaching English as a Second Language (TESL). Following two pairs of mentors and mentees, we analyze audio-recorded interactions and interviews and examine in depth how mediation occurs during mentoring and how such mediation assists the mentees in learning to become teachers. Our findings show that mentors' mediation is dynamic and contingent upon the context in which mentoring occurs and that the quality and character of mentor-mentee interactions during goal-directed activities is a critical feature of promoting the mentees' learning. Responsive mediation especially plays a key role in promoting the mentees' cognitive development, and it is an essential component of effective mentoring. This calls for a more SCT-informed approach to conceptualizing mentoring and training mentors in practicums.