Inclusion Criteria  Participants with histologically confirmed B-cell malignancy including:  Mantle cell lymphoma (MCL) treated with a prior Bruton's tyrosine kinase (BTK) inhibitor containing regimen; CLL/SLL treated with a prior BTK inhibitor containing regimen; Other types of B-cell NHL All participants must have disease requiring treatment, for CLL/SLL participants, at least 1 indication for treatment consistent with IWCLL 2018 criteria is required Eastern Cooperative Oncology Group 0-2 Adequate hematologic status, coagulation, hepatic and renal function  Exclusion Criteria  Lack of adequate wash-out period for investigational agent or anticancer therapy, major surgery, and radiotherapy prior to the first dose of study treatment Participants requiring therapeutic anticoagulation with warfarin Known central nervous system (CNS) involvement by systemic lymphoma. Primary CNS lymphoma is excluded Significant cardiovascular disease Prolongation of the QT interval Test positive for human immunodeficiency virus (HIV) Current treatment with certain strong cytochrome P450 3A4 (CYP450 3A4) inhibitors or inducers and/or strong p-glycoprotein (P-gp) inhibitors Pregnancy or lactation Active second malignancy Prior treatment with LOXO-305 Known hypersensitivity to any component or excipient of LOXO-305