The present study aims to design, implement and evaluate an online mentor training program in a pre-service language teacher education program. To this end, it consists of three phases. In the first phase, the expectations of university supervisors, mentors and student teachers from mentoring were investigated through questionnaires and interviews, which informed the content and design of the online mentor training program. In the second phase, the training program was implemented for a total of eight mentor teachers in the experimental group, whereas seven other mentors in the control group were engaged in usual practicum practices. The impact of program was investigated through the comparison of pre- and post-training interviews and surveys with two groups of mentors and their student teachers. In the third phase, the experimental group of mentors was asked to evaluate the designed training program for further improvement. The analysis and comparison of the responses given by the mentors and their student teachers showed that the online mentor training program made positive changes in mentors’ understanding of their roles and especially in their observation and feedback practices. In addition, the mentors who participated in the training program were contented with the presented content and tasks and made suggestions related to the timing of implementation as well as the delivery mode of the training.