Background: Lymphedema is a severe post-mastectomy complication that still causes much morbidity in breast cancer patients with axillary lymph node dissection. Therefore, after mastectomy, lymphedema prevention is crucial for long-term survival and an increase in quality of life. Aim: The study's primary objective was to determine whether pre-discharge educational intervention sessions for post-mastectomy women effectively improve the knowledge in preventing arm lymphedema. Methodology: A quasi-experimental research study, one-group pretest-posttest design, was conducted in a teaching hospital in Bangalore. The sample included 80 females who were diagnosed with breast cancer, had recently undergone mastectomy, and had chemo and radiation therapy plans. Questions were based on the knowledge of lymphedema preventive concepts, including the definition of the lymphatic system and lymphedema, its causes, symptoms, prevention, and management, and were included in a structured self-administered questionnaire. Convenience sampling was used among women who had undergone mastectomy for breast cancer. The pre-discharge educational interventions session included instructions on a self or simple lymphatic drainage technique demonstration, arm exercises, and an e-brochure on risk reduction strategies and arm lymphedema prevention. The knowledge of pre-discharge educational intervention sessions highlighting risk reduction/prevention strategies among post-mastectomy women was evaluated using the self-structured knowledge questionnaire pretest and posttest data. Results: The entire study population comprised women who had undergone mastectomy. Almost half of the subjects were older than 55 years. Prior to the intervention, the majority of patients (58) had poor knowledge (72%) about preventing lymphedema, whereas nearly all patients (80) had good knowledge (100%) after the intervention sessions. All participants felt comfortable using the treatment plan to avoid arm lymphedema. The knowledge gain was statistically significant at the 0.05 level. Conclusions: It was determined that pre-discharge educational intervention sessions enhanced post-mastectomy women's awareness and risk reduction behaviors toward preventing arm lymphedema and reducing arm morbidity. Therefore, it is suggested that women who have had breast cancer surgery participate in a pre-discharge educational intervention program. This will guarantee that all mastectomy patients have access to educational information/materials and that risk reduction strategies are followed to prevent lymphedema.