•Pembrolizumab achieved long term remission in recurrent high-grade microsatellite-stable endometrial carcinosarcoma.•This case challenges conventional treatment paradigms for microsatellite stable endometrial carcinosarcoma.•Insurance initially denied immunotherapy, highlighting barriers to access for rare malignancies.•This case demonstrates the need for further investigation of immune checkpoint inhibitors in aggressive endometrial cancers.•Expanding immunotherapy eligibility could improve outcomes for patients with limited treatment options.