Cardiac involvement in hematological malignancies is uncommon, with only a few cases reported to date, and it often leads to a poor prognosis. Here, we report a case of a 42-year-old woman with a history of allogeneic hematopoietic stem cell transplantation (allo-HSCT) and anti-CD19 chimeric antigen receptor (CAR) T-cell therapy for B-cell lymphoblastic lymphoma/acute lymphoblastic leukemia in whom cardiac mass and myocardial infiltration were detected. Prior to this presentation, massive pericardial effusion had occurred 6 months after CAR T-cell therapy, which was improved via ultrasound-guided pericardiocentesis. We observed elevated cytokine levels and increased copy number of CAR DNA in both pericardial effusion and serum. Upon detecting cardiac mass and myocardial infiltration, the patient was administered tocilizumab (a humanized monoclonal antibody against IL-6 receptor), which controlled the serum cytokine levels, and reduced intensity chemotherapy, including vindesine, cyclophosphamide, and prednisolone. However, the patient finally died of multiple organ failure. To the best of our knowledge, this is the first report on the development of a cardiac mass and occurrence of myocardial infiltration after allo-HSCT and CAR T-cell therapy. This report may provide supporting data for the early diagnosis and immediate treatment of patients with cardiac involvement.