Despite the critical role of mentors in the preparation of preservice teachers, very little causal quantitative research has explored the effects of offering mentors professional development (PD) around how to coach and support their candidates. We conduct an experimental evaluation of a PD opportunity randomly offered to mentors within six different Tennessee programs over the course of 3 years to investigate its impacts on the perceptions and practices of both the mentors themselves and their candidates. PD significantly increased the frequency of coaching provided by mentors and mentors' own instructional effectiveness but did not impact their candidates' satisfaction or feelings of preparedness, though candidates of mentors assigned to treatment were marginally significantly more likely to find employment.