Inclusion Criteria:

          -  Has a histologically confirmed new diagnosis of Endometrial Carcinoma or
             Carcinosarcoma (Mixed Mullerian Tumor) and:

               -  Has undergone curative intent surgery that included hysterectomy and bilateral
                  salpingo-oophorectomy; and

               -  Is at high risk for recurrence following treatment with curative intent surgery,
                  ie: Fédération Internationale de Gynécologie et d'Obstétrique (FIGO) 2009
                  surgical stage I/II with myometrial invasion of non-endometrioid histology; FIGO
                  2009 surgical stage I/II with myometrial invasion of any histology with known
                  aberrant p53 expression or p53 mutation; or FIGO (2009) surgical stage III or IVA
                  of any histology.

          -  Is disease-free with no evidence of loco-regional disease or distant metastasis post
             operatively and on imaging.

          -  Has not received any radiation or systemic therapy, including immunotherapy, hormonal
             therapy, or hyperthermic intraperitoneal chemotherapy (HIPEC), in any setting
             including the neoadjuvant setting for endometrial cancer (EC).

          -  Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 7
             days before randomization.

          -  Submission of a tumor tissue sample from current diagnosis of Endometrial Carcinoma or
             Carcinosarcoma for prospective determination of histology and mismatch repair (MMR)
             status by central vendor is required for all participants.

          -  Has adequate organ function within 7 days of randomization.

        Exclusion Criteria:

          -  Has recurrent endometrial carcinoma or carcinosarcoma.

          -  Has uterine mesenchymal tumor such as an endometrial stromal sarcoma, leiomyosarcoma,
             or other types of pure sarcomas. Adenosarcomas are also not allowed.

          -  Has FIGO (2009) Surgical Stage I/II EC of endometrioid histology without a known
             aberrant p53 expression or p53 mutation.

          -  Is known to have a deoxyribonucleic acid (DNA) polymerase epsilon catalytic subunit A
             (POLE) mutation.

          -  Has FIGO Stage IVB disease of any histology even if there is no evidence of disease
             after surgery.

          -  Has residual tumor whether measurable or non-measurable after surgery.

          -  Has a history of a second malignancy, unless potentially curative treatment has been
             completed with no evidence of malignancy for 3 years.

               -  Note: The time requirement does not apply to participants who underwent
                  successful definitive resection of basal cell carcinoma of the skin, squamous
                  cell carcinoma of the skin, superficial bladder cancer, in situ cervical cancer,
                  or other in situ cancers.

          -  Has received prior therapy with an anti-programmed cell death receptor 1 (PD-1),
             anti-programmed cell death receptor ligand 1 (PD-L1), or anti-programmed cell death
             receptor ligand 2 (PD-L2) agent or with an agent directed to another stimulatory or
             co-inhibitory T-cell receptor (eg, cytotoxic T-lymphocyte-associated protein 4
             (CTLA-4), OX-40, CD137).

          -  Has received a live vaccine within 30 days before the first dose of study
             intervention.

               -  Note: killed vaccines are allowed.

          -  Has a known intolerance to study intervention (or any of the excipients).

          -  Is currently participating in or has participated in a study of an investigational
             agent or has used an investigational device within 4 weeks before the first dose of
             study intervention.

               -  Note: Participants who have entered the follow-up phase of an investigational
                  study may participate as long as it has been 4 weeks after the last dose of the
                  previous investigational agent.

          -  Has any contraindication to the use of carboplatin or paclitaxel.

          -  Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy
             (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of
             immunosuppressive therapy within 7 days prior the first dose of study intervention.

          -  Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients.

          -  Has an active autoimmune disease that has required systemic treatment in past 2 years
             (ie, with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs). Replacement therapy (eg, thyroxine, insulin, or physiologic corticosteroid
             replacement therapy for adrenal or pituitary insufficiency) is not considered a form
             of systemic treatment and is allowed.

          -  Has a history of (non-infectious) pneumonitis that required steroids or has current
             pneumonitis.

          -  Has an active infection requiring systemic therapy.

          -  Has a known history of HIV infection.

          -  Has a known history of Hepatitis B or known active Hepatitis C virus infection.

          -  Has a known psychiatric or substance abuse disorder that would interfere with the
             participant's ability to cooperate with the requirements of the study.

          -  Has had an allogenic tissue/solid organ transplant.

          -  Has not recovered adequately from surgery and/or any complications from the surgery.

          -  Is breastfeeding.