This study evaluates the effectiveness of the professional development program designed for distance education instructors, in which design-based learning and mentoring strategies are implemented consecutively. Eight university instructors from the same university in Turkey with one year of experience in online teaching participated in the study. The study first conducted a needs analysis, that revealed that the instructors were not accustomed to student-centered instructional methods, and they maintained direct instruction in their online courses. Moreover, the instructors' technology integration understanding was inadequate, and they were generally concerned about the students' low participation in online classes. Considering these findings, the professional development program was designed for the instructors who first joined a one-week design-based learning training and then attended mentorship for two weeks. The findings indicated that this approach was effective for the instructors' professional development. The implications of this approach and suggestions for professional development programs were discussed based on the findings.