Inclusion Criteria - Participant has documented diagnosis of multiple myeloma and measurable disease, defined as any of the following:. i) Myeloma-protein (M-protein) ≥ 0.5 grams/deciliter (g/dL) by serum protein electrophoresis (sPEP), or. ii) M-protein ≥ 200 milligrams (mg)/24-hour urine collection by urine protein electrophoresis (uPEP) or,. iii) For participants without measurable disease in sPEP or uPEP: serum free light chain levels > 100 mg/liter (L) (10 mg/dL) involved light chain and an abnormal κ/λ free light chain ratio. - Participant has received at least one prior line of anti-myeloma therapy. Note: One line can contain several phases (e.g., induction, [with or without] hematopoietic stem cell transplant, (with or without) consolidation, and/or [with or without] maintenance therapy). - Participant must have received prior treatment with lenalidomide and at least 2 cycles of an anti-CD38 monoclonal antibody (mAb) (participants who were intolerant of an anti-CD38 mAb and received < 2 cycles are still eligible). - Participant achieved minimal response or better to at least 1 prior anti-myeloma therapy. - Participant must have documented disease progression during or after their last antimyeloma regimen. Exclusion Criteria - Participant who has had prior treatment with mezigdomide or carfilzomib. - Participant has previously received allogeneic stem cell transplant at any time or received autologous stem cell transplant within 12 weeks of initiating study treatment. - Other protocol-defined Inclusion/Exclusion criteria apply.