The qualification of a preschool teacher is one of the important factors that determine the quality of preschool education. To increase the quality in education, teachers should be equipped with the necessary knowledge starting from pre-service training, and in-service training should support the upgrading of their knowledge. Thus, the aim of this study was to develop an e-Mentoring-Based Education Program (e-MENTE:PT) for preschool teachers and to test its effectiveness on their professional development during the in-service support period. The study used a design-based research approach. First, focus group interviews were conducted with one-hundred-and-five preschool teachers to find out their needs. Then, an e-MENTE:PT Program was developed based on these needs and devised to be used on the Canvas learning management system (LMS), which involves all opportunities of e-learning and allows one-to-one e-mentoring. The e-MENTE:PT Program involved twelve modules related to the pedagogical content knowledge of the preschool education program. Each module included contents, goals, module materials, e-books, videos, pre- and post-module evaluation questions, discussion questions, live lessons and e-portfolios, all of which were integrated in the LMS. Next, eighteen full-time and nine substitute voluntary teachers from five different provinces were selected as mentees by means of criterion sampling. Nine academicians working in these provinces acted as mentors and implemented the application, each working with two mentees for two months. Qualitative data were analysed by means of descriptive analysis and quantitative data were analysed by means of the Wilcoxon signed rank test. The research concluded that the e-MENTE:PT program supported preschool teachers' professional knowledge, learning environments and classroom applications, and an e-mentoring based teaching management system was effective on professional development.