Relapsed/Refractory Arm

        Inclusion Criteria:

          -  Pathologically confirmed MCL (in tumor tissue), with documentation of either
             overexpression of cyclin D1 in association with other relevant markers (eg, CD19,
             CD20, PAX5, CD5) or evidence of t(11;14) as assessed by cytogenetics, fluorescent in
             situ hybridization (FISH), or polymerase chain reaction (PCR).

          -  At least 1 measurable site of disease on cross-sectional imaging (CT/PET).

          -  At least 1, but no more than 5, prior treatment regimens for MCL.

          -  Failure to achieve at least partial response (PR) with, or documented disease
             progression after, the most recent treatment regimen.

          -  Subjects must have adequate fresh or paraffin embedded tissue.

          -  Adequate hematologic, hepatic and renal function.

          -  Eastern Cooperative Oncology Group (ECOG) performance status (PS) of <= 2.

        Exclusion Criteria:

          -  History or current evidence of central nervous system lymphoma.

          -  Concurrent enrollment in another therapeutic investigational study or prior therapy
             with ibrutinib or other BTK inhibitors.

          -  Prior treatment with venetoclax or other BCL2 inhibitors.

          -  Anticancer therapy including chemotherapy, radiotherapy, small molecule and
             investigational agents 21 days prior to receiving the first dose of study drug.

          -  Treatment with any of the following within 7 days prior to the first dose of study
             drug: moderate or strong cytochrome P450 3A (CYP3A) inhibitors or strong CYP3A
             inducers.

        Treatment Naïve Arm

        Inclusion Criteria:

          -  Pathologically confirmed treatment-naive MCL (tumor tissue), with documentation of
             either overexpression of cyclin D1 in association with other relevant markers (eg,
             CD19, CD20, PAX5, CD5) or evidence of t(11;14), as assessed by cytogenetics,
             fluorescent in situ hybridization (FISH), or polymerase chain reaction (PCR).

          -  Men and women ≥18 years of age with a TP53 mutation.

          -  At least 1 measurable site of disease.

          -  Must have adequate fresh or paraffin-embedded tissue.

          -  Eastern Cooperative Oncology Group (ECOG) performance status score 0-2.

          -  Adequate hematologic, hepatic, and renal function.

        Exclusion Criteria:

          -  Blastoid variant of MCL

          -  History or current evidence of CNS lymphoma.

          -  Concurrent enrollment in another therapeutic investigational study or prior therapy
             including ibrutinib or other BTK inhibitors.

          -  Prior treatment with venetoclax or other BCL2 inhibitors.

          -  Vaccinated with live, attenuated vaccines within 4 weeks of the first dose of study
             drug.

          -  Clinically significant infection requiring IV systemic treatment that was completed
             <=14 days before the first dose of study drug.

          -  Any uncontrolled active systemic infection.

          -  Known bleeding disorders (eg, von Willebrand's disease or hemophilia).

          -  History of stroke or intracranial hemorrhage within 6 months prior to enrollment.

          -  History of HIV or active HCV or HBV.

          -  Major surgery within 4 weeks of the first dose of study drug.

          -  Any life-threatening illness, medical condition, or organ system dysfunction that, in
             the investigator's opinion, could compromise the participant's safety or put the study
             outcomes at undue risk.

          -  Currently active, clinically significant cardiovascular disease; or a history of
             myocardial infarction, unstable angina, or acute coronary syndrome within 6 months
             prior to randomization.

          -  Unable to swallow capsules or tablets, or malabsorption syndrome, disease
             significantly affecting gastrointestinal function, or resection of the stomach or
             small bowel, symptomatic inflammatory bowel disease or ulcerative colitis, or partial
             or complete bowel obstruction.

          -  Treatment with any of the following within 7 days prior to the first dose of study
             drug: Moderate or strong cytochrome P450 3A (CYP3A) inhibitors or moderate or strong
             CYP3A inducers.

          -  Known allergy to xanthine oxidase inhibitors and/or rasburicase for subjects with
             known risk factors (as defined by high tumor burden and/or diminished renal function,
             as detailed in "Study Design" section above) for TLS.

          -  Chronic liver disease with hepatic impairment Child-Pugh class B or C.

          -  Unwilling or unable to participate in all required study evaluations and procedures.

          -  Known hypersensitivity to the active ingredient or other components of one or more
             study drugs.