A (La,Mg)5Ni19 single-phase La0.78Mg0.22Ni3.73 alloy with superior overall electrochemical properties has been prepared by step-wise annealing the as-cast alloy sample. The as-cast alloy is composed of 2H-/3R-type (La,Mg)2Ni7 and (La,Mg)5Ni19 main phases as well as (La,Mg)Ni3 and LaNi5 minor phases. During step-wise annealing process, (La,Mg)Ni3 and LaNi5 minor phases are first consumed to form (La,Mg)2Ni7 and (La,Mg)5Ni19 phases at 800–850 °C. Then the 3R-type (La,Mg)2Ni7 phase transforms to (La,Mg)5Ni19 phase at 900 °C, followed by the transformation of 2H-type (La,Mg)2Ni7 phase to (La,Mg)5Ni19 phase which is completed at 950 °C, producing the alloy with (La,Mg)5Ni19 single phase. The (La,Mg)5Ni19 single-phase alloy possesses good structural stability and strong resistance against pulverization and corrosion/oxidation during repeated charge/discharge cycling, achieving a cycle life of above 400 cycles, nearly three times that of the as-cast alloy. Meanwhile, the (La,Mg)5Ni19-phase alloy electrode exhibits outstanding kinetics performance with a high rate dischargeability of 62.4% at 1500 mA g−1 compared with 51.2% of the as-cast alloy. The (La,Mg)5Ni19 single-phase alloy is considered a promising potential electrode material for high-power and long-lifetime Ni/MH batteries.