Chimeric antigen receptor natural killer (CAR-NK) cell therapy is recognized as a promising modality for the treatment of hematologic malignancies, particularly B-cell malignancies. In this study, we developed “off-the-shelf” anti-CD19 CAR-NK cells using anti-CD19 CAR mRNAs formulated in proprietary ionizable lipid nanoparticles (LNPs). The efficiency of mRNA-LNP delivery into umbilical cord blood (UCB)-derived NK cells and primary T cells was evaluated in an in vitro setting, demonstrating superior delivery efficiency in NK cells. Further investigation showed a probable role for an endocytic mechanism, macropinocytosis, in efficient transfection of NK cells with LNPs. Nevertheless, CAR-NK cells generated through this mRNA-LNP platform exhibited significantly enhanced cytotoxicity against CD19 ⁺ target cells, such as EGFP ⁺ Raji stable cell line and primary malignant B cells derived from refractory/relapsed B-cell acute lymphoblastic leukemia (B-ALL) patients. These findings highlight the promise of the mRNA-LNP platform in advancing CAR-NK therapies against B-cell malignancies.