Abstract Differential diagnoses of cardiac masses include primary benign and malignant neoplasms, secondary neoplasms, and non-neoplastic masses, such as thrombi. Owing to different therapeutic approaches and the way these affect the prognosis, the early and correct diagnostic determination of the etiology of a cardiac mass is of utmost importance and essential for the appropriate management of patients. We report a case of a 52-year-old woman with a left ventricular mass in the setting of a recent Mycoplasma pneumoniae infection and a medical history of cutaneous T-cell lymphoma and hypereosinophilia. Imaging findings were consistent with both an infiltrative process of the lymphoma and a cardiac thrombus. An estimated very high risk for embolization led to the indication for open-heart surgery for the removal of the cardiac mass. Histopathological examination confirmed the presence of a thrombus; there were no signs of malignancy. The patient was discharged 11 days after surgery in good general condition and is now in outpatient care for follow-up and further management. This case highlights possible challenges in the diagnostic assessment of cardiac masses and their management in a patient with several underlying diseases and a complex medical history.