A 48-year-old woman was diagnosed with synchronous mixed clear cell carcinoma of ovarian origin and endometroid endometrial carcinoma after presenting with intermenstrual bleeding for 2 years prior. Shortly after diagnosis she became progressively unwell requiring intensive care unit admission with respiratory failure, pleural effusions and pulmonary emboli. Following a total abdominal hysterectomy, bilateral salpingo-oophorectomy, laparotomy and emergency percutaneous thrombectomy, she remained critically unwell and was deemed not safe for chemotherapy. Given a high index of suspicion for Lynch syndrome, the patient was treated with adjuvant pembrolizumab and achieved a complete response. Lynch syndrome was subsequently confirmed through germline genetic testing. The patient made an excellent recovery and remains disease-free at 23 months.