Students with intellectual and developmental disabilities have more opportunity to attend postsecondary education than ever before. Peer mentors who support these students sometimes need to manage challenging behavior. This study examined the impact of training and coaching with performance feedback on peer mentors' fidelity in implementing function-based intervention plans. A nonconcurrent single subject multiple baseline design across three peer mentor-student pairs was used. All peer mentors improved their fidelity of implementing student behavior plans immediately after being trained and further improved after being coached. One to two coaching sessions were necessary for peer mentors to reach high levels in implementation fidelity. Students also decreased problem behaviors and increased prosocial behaviors when function-based support was implemented with fidelity. Study implications, limitations, and suggestions for future research are presented.