The main inclusion and exclusion criteria include but are not limited to the following:  Inclusion Criteria:  Has a histologically confirmed diagnosis of Stage III or IV or recurrent Endometrial Carcinoma (EC) or carcinosarcoma (mixed Mullerian tumor) that is centrally confirmed as dMMR  Has received no prior systemic therapy for advanced EC except for the following:  May have received prior radiation with or without radiosensitizing chemotherapy if >2 weeks before the start of study intervention. Participants must have recovered from all radiation-related toxicities, not require corticosteroids, and not have had radiation pneumonitis. A 1-week washout is permitted for palliative radiation (≤2 weeks of radiotherapy) to non-central nervous system (CNS) disease May have received prior hormonal therapy for treatment of EC, provided that it was discontinued ≥1 week prior to randomization Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 7 days before randomization Is not pregnant or breastfeeding and agrees to not donate eggs and use a highly effective contraceptive method for 120 days after the last dose of pembrolizumab or 180 days after the last dose of chemotherapy if a woman of childbearing potential (WOCBP) Has a negative highly sensitive pregnancy test (urine or serum) within 24 hours for urine or 72 hours for serum before the first dose of study intervention if a WOCBP Provides an archival tumor tissue sample or newly obtained (core, incisional, or excisional) biopsy of a tumor lesion not previously irradiated for verification of dMMR status and histology Is Hepatitis B surface antigen (HBsAg) positive but has received Hepatitis B virus (HBV) antiviral therapy for at least 4 weeks and has undetectable HBV viral load prior to randomization Has a history of Hepatitis C virus (HCV) infection but has undetectable HCV viral load at screening  Exclusion Criteria:  Has uterine mesenchymal tumor such as an endometrial stromal sarcoma, leiomyosarcoma, or other types of pure sarcomas. Adenosarcomas and neuroendocrine tumors are not allowed Has EC of any histology that is proficient mismatch repair (pMMR) Is a candidate for curative-intent surgery or curative-intent radiotherapy Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent or with an agent directed to another stimulatory or coinhibitory T-cell receptor (e.g. cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], Tumor necrosis factor receptor superfamily, member 4 [OX 40], tumor necrosis factor receptor superfamily member 9 [CD137]) Has received prior systemic anticancer therapy including investigational agents for EC. This includes any chemotherapy given for EC other than as a radiosensitizer Has had a major operation and has not recovered adequately from the procedure and/or any complications from the operation before starting study intervention Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention. Administration of killed vaccines are allowed Is currently participating in or has participated in a study of an investigational agent for EC, has participated in a study of an investigational agent for non-EC within 4 weeks before the first dose of study intervention, or has used an investigational device within 4 weeks before the first dose of study intervention Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study intervention Has a known additional malignancy that is progressing or has required active treatment within the past 3 years with the exception of basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ (excluding carcinoma in situ of the bladder) that have undergone potentially curative therapy are not excluded Has known active CNS metastases and/or carcinomatous meningitis Has a known intolerance to any study intervention and/or any of its excipients Has an active autoimmune disease that has required systemic treatment in past 2 years Has a history of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease Has an active infection, requiring systemic therapy Has a known history of human immunodeficiency virus (HIV) infection Has had an allogenic tissue/solid organ transplant