Cancers of unknown primary (CUPs) pose diagnostic and therapeutic challenges because of their aggressive nature and elusive identification through conventional diagnostic means. This case report presents a unique finding in a 57-year-old female patient who, alongside her CUP, developed a cavernous mesenteric lymphangioma—a rare lymphatic malformation that is more commonly observed in pediatric populations. The interrelation between CUPs, chylous ascites, and lymphangiomas is explored, shedding light on this uncommon occurrence. Our patient’s journey began with a CUP diagnosis, marked by metastatic adenocarcinoma. Subsequent symptoms led to the discovery of a mesenteric lymphangioma. The case unfolds the patient’s diagnostic challenges, surgical interventions, and postoperative course. By highlighting the distinctive features of our case, we aim to contribute to the limited understanding of the interplay between lymphatic malformations and metastatic malignancies.