Advanced endometrial cancer is associated with poor outcomes and few treatment options exist. Recently, the US Federal Drug Administration approved pembrolizumab for the treatment of endometrial cancers that are deficient in mismatch repair and have high microsatellite instability (MSI). Lynch syndrome is an autosomal dominant disease that causes MSI-high endometrial cancer. We report a case of a 46-year-old woman with Lynch syndrome and advanced endometrial cancer who experienced progressive disease after treatment with chemotherapy with carboplatin and paclitaxel. She was then treated with single-agent pembrolizumab and had an exceptional response. She was noted to have a significant decrease in the size of a large uterine mass extending into the vagina and vulva, as well as decrease in the size of lymphadenopathy. Data are limited at this time for patients with Lynch syndrome treated with single-agent pembrolizumab. Our case report seeks to add to the body of literature that suggests that this patient population may particularly benefit from this novel therapy.