Background and Clinical Significance: Mantle cell lymphoma (MCL) frequently involves bone marrow, gastrointestinal tract, and hepatosplenomegaly, whereas pleural effusions are uncommon. Cases requiring invasive mechanical ventilation and thoracic drainage are rare. We report a case of MCL with persistent massive pleural effusions requiring invasive mechanical ventilation and bilateral continuous thoracic drainage. Case Presentation: A 71-year-old woman presented with dyspnea and was found to have bilateral pleural effusions and generalized lymphadenopathy. Shortly after admission, she developed acute respiratory failure due to pleural effusions and required invasive mechanical ventilation. Right-sided continuous thoracic drainage was initiated. Thereafter, more than 1 L of pleural fluid was drained each day. Flow cytometry of the pleural fluid showed CD5-positive B cells with kappa light-chain restriction. Bone marrow examination revealed abnormal lymphocyte infiltration. Cervical lymph node biopsy demonstrated diffuse proliferation of medium-sized, abnormal B lymphocytes with an immunophenotype of CD5⁺, CD19⁺, CD20⁺, cyclin D1⁺, SOX11⁺, and κ⁺, with a Ki-67 index of 20%, confirming MCL, stage IV. Immunochemotherapy with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) was commenced under mechanical ventilation. Shortly thereafter, left-sided continuous thoracic drainage was also initiated. However, in response to immunochemotherapy, the bilateral pleural effusions gradually subsided, enabling extubation, and there was no reaccumulation after removal of both chest tubes. Furthermore, generalized lymphadenopathy regressed, and bone marrow examination revealed resolution of lymphoma infiltration, resulting in complete remission. Conclusions: De novo MCL complicated by persistent massive pleural effusions requiring invasive mechanical ventilation and bilateral continuous thoracic drainage is rare. A thorough diagnostic workup followed by prompt initiation of immunochemotherapy can arrest pleural output, enable extubation, and be lifesaving. Clinicians should recognize that MCL rarely presents with persistent massive pleural effusions.