Samples with compositions of 95 wt% Mg + 5 wt% CMC(Na) [carboxymethylcellulose, sodium salt, {C6H7O₂(OH)x(C₂H₂O₃Na)y}n] [named Mg-5CMC(Na)] and 90 wt% Mg + 10 wt% CMC(Na) [named Mg-10CMC(Na)] were prepared via milling in hydrogen (hydride-forming milling). Mg-5CMC(Na) and Mg-10CMC(Na) had very high hydrogenation rates but low dehydrogenation rates. Adding Ni to Mg is known to increase the hydrogenation and dehydrogenation rates of Mg. We chose Ni as an additive to increase dehydrogenation rates of Mg-5CMC(Na) and Mg-10CMC(Na). A sample with a composition of 90 wt% Mg + 5 wt% CMC + 5 wt% Ni [named Mg-5Ni-5CMC(Na)] was prepared via hydride-forming milling. The activation of Mg-5Ni-5CMC(Na) was completed at the third hydrogenation-dehydrogenation cycle (N ═ 3). Mg-5Ni-5CMC(Na) had an effective hydrogen-storage capacity (the quantity of hydrogen absorbed for 60 min) of 5.83 wt% at 593 K in 12 bar hydrogen at N ═ 3. Mg-5Ni-5CMC(Na) released 2.73 wt% H for 10 min and 4.61 wt% H for 60 min at 593 K in 1.0 bar hydrogen at N ═ 3. Mg-5Ni-5CMC(Na) dehydrogenated at N ═ 4 contained Mg and small amounts of MgO, β-MgH₂, Mg₂Ni, and Ni. Hydride-forming milling of Mg with CMC and Ni and Mg₂Ni formed during hydrogenation-dehydrogenation cycling are believed to have increased the dehydrogenation rates of Mg-5CMC(Na) and Mg-10CMC(Na). As far as we know, this study is the first in which a polymer CMC(Na) and Ni were added to Mg via hydride-forming milling to improve the hydrogenation and dehydrogenation rates of Mg.