Ataxia telangiectasia (AT) is a rare autosomal recessive disorder caused by the pathological variants of the ATM gene. Owing to i ts r arity a nd n ature, complications of AT, such a s malignant tumors, a re often difficult to manage with standard imaging studies and treatments, and there are no established management strategies. We report the case of a woman who had AT in childhood and developed breast cancer in her 20s; the disease was successfully managed by the decision-making of multidisciplinary physicians professionals with ethics support. She was immunocompromised, ataxic, and mentally impaired. The patient's mother noticed a tumor in her right breast and subsequently brought her to our department. Although preoperative testing and surgical procedures were limited as AT is extremely radiosensitive, the patient was diagnosed with cT2N0M0 breast cancer and underwent right mastectomy and axillary lymph node sampling. The final diagnosis was pT2N0M0 pStage IIA mucinous carcinoma, and immunohistochemistry of the tumor specimen was estrogen receptor-positive, progesterone receptor-positive, and HER2-negative. Tamoxifen was administered as postoperative adjuvant therapy, and the patient has survived to date without recurrence. Here, we report our experience with breast cancer treatment for AT, along with a review of the literature.