We encountered a patient with composite mantle cell lymphoma (MCL) and T-cell prolymphocytic leukemia (T-PLL) who presented with inactive disease to active T-PLL over 8 years. A 71-year-old man was diagnosed with MCL with an atypical T-cell population showing CD2+, CD3-, CD4+, CD7+, CD8-, and CD25+; however, the cause of the T-cell population could not be determined at the first MCL diagnosis. When MCL relapsed approximately 8 years after the initial treatment, T-PLL was definitively diagnosed using the T-PLL International Study Group criteria. MCL and T-PLL were determined to coexist in the lymph nodes and bone marrow by histological or flowcytometry analysis. Retrospective flow cytometry and T-cell receptor-polymerase chain reaction analysis of the stored samples suggested that the T-cell population noted at the time of initial MCL diagnosis eight years earlier was the same clone of T-PLL and the progression from inactive disease to active disease of his T-PLL. To the best of our knowledge, this is the first report of a composite MCL and T-PLL.