Historically, endometrial carcinomas have been classified primarily according to their histology. However, the use of immunohistochemistry has become commonplace in their evaluation, particularly in diagnostically challenging cases. Our objective was to evaluate mixed endometrial carcinomas using a well-established panel of biomarkers to assess the consistency and utility of these stains in clinical diagnosis. Eighteen cases comprised of various combinations of classical serous (SC), endometrioid (EC), and clear cell (CC) morphologies were identified and subjected to a panel of immunohistochemical markers including p53, p16, Ki67, estrogen receptor, progesterone receptor, and Napsin A. Intensity and extent of staining were evaluated on 4-tiered and 5-tiered scales, respectively. The typical immunostaining pattern expected for the individual tumor components was seen in only 3 cases, while in 15 cases an unexpected pattern was observed with at least one immunomarker. By tumor type, the most common unexpected finding in EC/SC carcinoma cases was diffuse positivity for p16 and/or estrogen receptor/progesterone receptor in both components, while in SC/CC, diffuse positivity for p53 in both components was most frequently seen, and in SC/CC/EC, Napsin A negativity was most commonly observed. Despite displaying diagnostic morphology, components of many mixed endometrial carcinomas may not exhibit expected immunohistochemical features. This may be due to the fact that these carcinomas arise from a single clone with subsequent divergence, resulting in a tumor with both mixed histologic and genetic features. It is important to note that these tumors may not demonstrate the immunohistochemical prototype of their constituents and should be approached accordingly from a diagnostic perspective.