Herein, we report the case of a 56-year-old man with composite lymphoma (CL) comprised of mantle cell lymphoma (MCL) and follicular lymphoma (FL). Six months after developing a right brachial tumor, he was diagnosed as having grade 3 FL with normal-size mantle zone. Simultaneously, advanced stage MCL with a diffuse growth pattern in a sigmoid colon tumor and abnormal lymphoid cells in bone marrow were observed. Thereafter, the right brachial tumor was re-examined and its mantle zone cells were immunophenotypically positive for cyclin D1 (CCND1) and cytogenetically positive for the IgH-CCND1 fusion gene. Consequently, he was diagnosed with composite lymphoma (CL) comprised of FL and MCL. As MCL and FL may form CL, the possible complication of MCL should be considered and steps taken to detect MCL.