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Abstract Samples with compositions of 95 wt% Mg + 5 wt% CMC (Carboxymethylcellulose, Sodium Salt) (named Mg-5CMC) and 90 wt% Mg + 10 wt% CMC (named Mg-10CMC) were prepared through milling in a hydrogen atmosphere (transformation-involving milling). After activation, Mg-5CMC had a larger amount of hydrogen absorbed in 60 min, U (60 min), than Mg-10CMC and milled Mg at 593 K. At the cycle number (CN) of four, Mg-5CMC had a very high beginning hydrogen uptake rate (1.45 wt% H/min) and a very large effective hydrogen-storage capacity of about 7.4 wt%. Mg-5CMC released 4.76 wt% H in 60 min at 648 K in hydrogen of 1.0 bar. It is believed that CMC melted during milling, and that since CMC has high viscosity, sliding between Mg particles and hardened steel balls was prevented, leading to effective milling (generation of defects and cracks and reduction of particle size). As far as we know, this study is the first in which a polymer CMC was added to Mg to improve the hydrogen uptake and release properties of Mg. Graphical abstract Image 1 Highlights • Preparation of a polymer Carboxymethylcellulose (CMC)-added Mg alloy by milling in hydrogen. • Hydrogen-storage capacity of about 7.4 wt% by activated Mg-5CMC at 593 K in hydrogen of 12 bar. • A very high initial hydrogen uptake rate of 1.45 wt% H/min at 593 K in 12 bar hydrogen after activation. • Addition of CMC is more effective than additions of halogen compounds, oxides, or metals. • First study where CMC is added by milling in hydrogen to improve hydrogen-storage properties of Mg. [ABSTRACT FROM AUTHOR]