Objective: The antitumor effects of anti-PD-1 antibody against mismatch repair deficiency (MMR-D)-associated cancers have been reported. MMR-D is found in approximately 20%–30% of endometrial carcinomas (ECs) and frequently occurs due to MLH1 promoter hypermethylation (MLH1-PHM). ECs with MLH1-PHM are classified according to the molecular screening of Lynch syndrome (LS), but few detailed reports are available. The purpose of this study was to clarify the clinical features of EC with MLH1-PHM. Methods: Immunohistochemistry of MMR proteins (MLH1, MSH2, MSH6, and PMS2) was performed on specimens from 527 ECs treated at our university hospital from 2003 to 2018. MLH1 methylation analysis was added to cases with MLH1/PMS2 loss. ECs were classified as follows: cases that retained MMR proteins as “MMR-proficient;” cases with MLH1/PMS2 loss and MLH1-PHM as “met-EC;” and cases with other MMR protein loss and MLH1/PMS2 loss without MLH1-PHM as “suspected-LS.” The clinical features, including long-term prognosis, of each group, were analyzed. Results: Accordingly, 419 (79.5%), 65 (12.3%), and 43 (8.2%) cases were categorized as “MMR-proficient,” “suspected-LS,” and “met-EC,” respectively. Significantly, “met-EC” had a lower proportion of grade 1 tumors (37.5%) and a higher proportion of stage III/IV tumors (37.2%) than the other groups. The overall and progression-free survival of “met-EC” were significantly worse than those of “suspected-LS” in all cases. Conclusion In ECs with MMR-D, “met-ECs” were a subgroup with a poorer prognosis than “suspected-LS.” “Met-ECs” would be the main target for anti-PD-1 antibody treatment, and its clinical susceptibility should be verified individually.