Background: Exercise following mastectomy is crucial for female breast cancer patients' recuperation from both the procedure and radiation therapy. It helps them keep their arm and shoulders flexible and reduces adverse effects like arm lymphedema and radiation-induced fibrosis. These exercises can improve arm and shoulder range of motion and are often initiated within a few days. These women need specialized educational intervention to improve their knowledge and practices regarding post-mastectomy exercise and decrease side effects after mastectomy. Aim: To determine the effectiveness of mobile-based educational exercises on women's knowledge, practices, and arm Lymphedema degree post-mastectomy. Design: quasi quasi-experimental (one group pre-posttest) research design was utilized in the study.  Setting: The research was conducted at the outpatient clinic of Sohag Oncology Institution. Subjects: A purposive sample of 100 post-mastectomy women was included. Tools: Tool I: structured interviewing questionnaire, it included three parts: (1) demographic data; (2) medical data, (3) clinical manifestations of lymphedema, Tool II: Women's knowledge regarding mobile based-educational exercises post-mastectomy (pre/post) and Tool III: Women's practices regarding mobile based-educational exercises post-mastectomy (pre/post). Results: According to the study's findings, doctors were the primary source of information for women who had had mastectomy surgery. Following the mobile-based educational exercises, women's knowledge and practices were improved. The post-mastectomy women's practice scores and overall knowledge showed a statistically significant correlation. There was a significant improvement in the degree of arm lymphedema post the mobile educational exercises. Conclusion: A mobile-based educational exercise was effective in improving women's knowledge, practices, and arm lymphedema degree post-mastectomy. Recommendations: To increase post-mastectomy exercise knowledge and practices, women who have had mastectomy surgery can benefit from mobile-based educational techniques.