The study focused on the improvement of supervisory skills among mentor teachers in K-12 who work with student teachers in an English language teaching program of an English-medium state university in Istanbul, Turkey. The researchers explored how the mentor teachers' supervisory skills, namely observation and giving feedback, were improved after participating in an online mentor training program. The training program was designed based on Morrison et al.'s (2013) instructional design model and implemented for seven mentors during the practicum period. The program included mentoring tasks that asked mentor teachers to organize reciprocal observation sessions with the student teachers, followed by mutual feedback discussions. The researchers investigated the influence of the tasks through thematic analysis of the mentor teachers' reflection reports. The data showed that the tasks improved mentor teachers' understanding and practice of the supervision process and increased their awareness of their own teaching practices by promoting personal and professional growth. The results suggest that an online mentor training program may have the potential to empower mentor teachers both for their supervisory roles and ongoing professional development.