Inclusion Criteria: 1. Men and women ≥ 18 years of age, at the time of signing the ICD (Informed Consent Document). 2. Subject must understand and voluntarily sign an ICD prior to any study-related assessments/procedures being conducted. 3. Relapsed or refractory AML (Acute Myeloid Leukemia) (Parts A and B) or relapsed or refractory high-risk MDS (Myelodysplastic Syndrome) (Part B only) as defined by World Health Organization criteria who are not suitable for other established therapies. 4. Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0 to 2. 5. At least 4 weeks (from first dose) has elapsed from donor lymphocyte infusion (DLI) without conditioning. 6. Subjects must have the following screening laboratory values: - Total White Blood Cell count (WBC) < 25 x 109/L prior to first infusion. Prior or concurrent treatment with hydroxyurea to achieve this level is allowed. - Selected electrolytes within normal limits or correctable with supplements. - Serum bilirubin ≤ 1.5 x ULN (upper limit of normal). - Estimated serum creatinine clearance of ≥ 60 mL/min using the Cockcroft-Gault equation. 7. Agree to follow the CC-90009 Pregnancy Prevention Plan (PPP) Exclusion Criteria: 1. Subjects with acute promyelocytic leukemia (APL) 2. Subjects with clinical symptoms suggesting active central nervous system (CNS) leukemia or known CNS leukemia. Evaluation of cerebrospinal fluid is only required if there is clinical suspicion of CNS involvement by leukemia during screening. 3. Patients with prior autologous hematopoietic stem cell transplant who, in the investigator's judgment, have not fully recovered from the effects of the last transplant (e.g., transplant related side effects). 4. Prior allogeneic hematopoietic stem cell transplant (HSCT) with either standard or reduced intensity conditioning ≤ 6 months prior to starting CC-90009. 5. Subjects on systemic immunosuppressive therapy post HSCT at the time of screening, or with clinically significant graft-versus-host disease (GVHD). The use of topical steroids for ongoing skin or ocular GVHD is permitted. 6. Prior systemic cancer-directed treatments or investigational modalities ≤ 5 half lives or 4 weeks prior to starting CC-90009, whichever is shorter. Hydroxyurea is allowed to control peripheral leukemia blasts. 7. Leukapheresis ≤ 2 weeks prior to starting CC-90009. 8. For Part B, previous SARS-CoV-2 infection within 10 days for mild or asymptomatic infections or 20 days for severe/critical illness prior to C1D1. 9. For Part B, previous COVID-19 vaccine within 14 days of C1D1.