Mantle cell lymphoma (MCL) is a B-cell non-Hodgkin lymphoma with an advanced stage; it occurs frequently and affects the lymph nodes, spleen, blood and bone marrow. The synchronous occurrence of MCL bone marrow involvement (MCLBMI) and malignant tumors is extremely rare. To the best of our knowledge, synchronous extensive-stage small cell lung cancer (ES-SCLC) and MCLBMI have not been previously reported. In the present study, a rare case of ES-SCLC with synchronous MCLBMI is reported in a 59-year-old man. The patient received cisplatin, etoposide, dexamethasone and rituximab chemotherapy for the treatment of both malignancies. The follow-up computed tomography scan disclosed regression of the left upper lobe mass and the routine blood test indicated that the platelet count was gradually increasing to normal levels. Following therapy, the patient achieved a partial response. The experience in this case report indicated that the treatment of synchronous SCLC and MCLBMI requires consideration of the respective patient clinical features, biological behavior and cumulative toxicity of the treatment regimens administered for both malignant tumors. The present study demonstrated that thrombocytopenia was not a chemotherapy contraindication, thus providing a new treatment option for this type of patient.