Inclusion Criteria:

* Patients must have:

* Stage I endometrioid endometrial cancer and a combination of age and risk factors as listed below:

* Age >= 70 and 1 or more risk factors
* Age 50 - < 70 and 2 or more risk factors
* Age < 50 and 3 risk factors

* Risk factors:

* Myometrial invasion >= 50%
* Lymphovascular space invasion
* Grade 2 or 3 OR
* Stage II endometrioid endometrial cancer

* Note: Patients with isolated tumor cells in sentinel lymph nodes are eligible (considered N0i) as long as there is no evidence of micro- or macro-metastases in any lymph nodes
* CT or MRI abdomen or pelvis and either chest X-ray or CT chest demonstrating no evidence of disease outside of the uterus. Imaging can be performed pre-operatively or post-operatively. CT with contrast is the preferred modality. Positron emission tomography (PET)/CT is NOT to be used for any disease assessment or reassessment unless there is documentation that PET/CT is of diagnostic quality equal to CT with contrast
* Patients must have deficient mismatch repair as demonstrated by lack of expression of at least one mismatch repair protein by immunohistochemistry (IHC) and/or evidence of microsatellite instability (MSI) high. The institutional pathology report documenting MMR deficiency must be submitted
* Patients must have undergone surgical staging with at least hysterectomy, removal of cervix, bilateral (if both are present) salpingo-oophorectomy, and either sentinel lymph node assessment or complete pelvic +/- aortic lymphadenectomy. Secondary staging is allowed to determine stage. Patients with isolated tumor cells in sentinel lymph nodes are eligible (considered N0i) as long as there is no evidence of micro- or macro-metastases in any lymph nodes
* Patients must have received no prior therapy for endometrial cancer, including hormonal therapy, chemotherapy, targeted therapy, immunotherapy or radiation therapy
* Age >= 18
* Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2
* Platelets >= 100,000/mcl (within 14 days prior to registration)
* Absolute neutrophil count (ANC) >= 1,500/mcl (within 14 days prior to registration)
* Creatinine =< 1.5 x laboratory upper limit of normal (ULN) (within 14 days prior to registration)
* Bilirubin =< 1.5 x ULN (within 14 days prior to registration) (patients with known Gilbert's disease who have bilirubin level =< 3 x ULN may be enrolled)
* Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) =< 3 x ULN (within 14 days prior to registration)
* Thyroid stimulating hormone (TSH) within normal limits (TSH < ULN allowed in euthyroid patients on thyroid replacement therapy)
* Patients must be registered between 1 and 8 weeks after initial (staging) surgery performed for the combined purpose of diagnosis and staging
* Human immunodeficiency virus (HIV) testing is not required by protocol unless clinically indicated. Known HIV positive patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial
* Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial
* The patient or a legally authorized representative must provide study-specific informed consent prior to study entry and, for patients treated in the United States (U.S.), authorization permitting release of personal health information

Exclusion Criteria:

* Patients who are currently participating and receiving cancer-directed study therapy for endometrial cancer or have participated in a study of an investigational agent and received cancer-directed study therapy for endometrial cancer within 4 weeks prior to registration
* Patients who have received prior treatment with anti-PD-1, anti-PD-L1, or anti-CTLA-4 therapeutic antibody or other similar agents
* Patients who have a history of a severe hypersensitivity reaction to monoclonal antibody or MK-3475 (pembrolizumab) and/or its excipients
* Patients with active autoimmune disease or history of autoimmune disease that might recur, which may affect vital organ function or require immune suppressive treatment including systemic corticosteroids. This includes, but is not limited to, patients with a history of immune related neurologic disease, multiple sclerosis, autoimmune (demyelinating) neuropathy, Guillain-Barre syndrome, myasthenia gravis; systemic autoimmune disease such as systemic lupus erythematosus (SLE), connective tissue diseases, scleroderma, inflammatory bowel disease (IBD), Crohn's, ulcerative colitis, hepatitis; and patients with a history of toxic epidermal necrolysis (TEN), Stevens-Johnson syndrome, or phospholipid syndrome because of the risk of recurrence or exacerbation of disease. Patients with vitiligo, endocrine deficiencies including type I diabetes mellitus, thyroiditis managed with replacement hormones including physiologic corticosteroids are eligible. Patients with rheumatoid arthritis and other arthropathies, Sjogren's syndrome and psoriasis controlled with topical medication and patients with positive serology, such as antinuclear antibodies (ANA), anti-thyroid antibodies should be evaluated for the presence of target organ involvement and potential need for systemic treatment but should otherwise be eligible
* Patients with a history of (non-infectious) pneumonitis that required steroids, or current pneumonitis
* Patients who have a diagnosis of immunodeficiency or are receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to registration:

* Patients who have received steroids as CT scan contrast premedication may be enrolled
* The use of inhaled or topical corticosteroids is allowed
* The use of mineralocorticoids (e.g., fludrocortisone) for patients with orthostatic hypotension or adrenocortical insufficiency is allowed
* The use of physiologic doses of corticosteroids may be approved after consultation with the study chair (e.g. 10 mg of prednisone used for replacement therapy for adrenal insufficiency)
* Patients who are lactating
* Known clinically significant liver disease, including active viral, alcoholic, or other hepatitis; and cirrhosis. For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral load must be undetectable on suppressive therapy, if indicated. Patients with a history of hepatitis C virus (HCV) infection must have been treated and cured. For patients with HCV infection who are currently on treatment, they are eligible if they have an undetectable HCV viral load
* Uncontrolled intercurrent illness including, but not limited to: ongoing or active infection (except for uncomplicated urinary tract infection), interstitial lung disease or active, non-infectious pneumonitis, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements
* Patients who have received any of the prohibited medications