CoP is considered as an efficient electrocatalyst for the hydrogen evolution reaction (HER) in acidic electrolytes but its performance in alkaline solutions is generally poor because of its slow reaction kinetics, which further limits its application in overall water splitting. Herein, we demonstrate a strategy to greatly accelerate its HER and OER kinetics in alkaline solutions through Mn incorporation. Ternary MnxCo1−xP microcubes with a tunable Mn/Co ratio strongly anchored on rGO were synthesized using Prussian blue analogues as precursors. The synergy between the high activity of MnxCo1−xP microcubes and the good conductivity of rGO leads to the superior performance of the hybrid toward water splitting in 1 M KOH. The optimized Mn0.6Co0.4P–rGO electrocatalyst shows high activity and stability towards both the HER and OER with low overpotentials of 54 and 250 mV at 10 mA cm−2, respectively. Furthermore, the water electrolyzer using Mn0.6Co0.4P–rGO as both the cathode and anode only requires a cell voltage as low as 1.55 V to reach a current density of 10 mA cm−2, making Mn0.6Co0.4P–rGO a competitive and cost-effective electrocatalyst for water splitting.