Water splitting is considered as a potential sustainable and green technology for producing mass hydrogen and oxygen. A cost-effective self-supported stable electrocatalyst with excellent electrocatalytic performance in a wide pH range is greatly required for water splitting. This work reports on the synthesis and anchoring of Fe1CoxNiyP nanoparticles on vertically aligned reduced graphene oxide array (VrGO) via electroless plating. The catalytic activity of Fe1CoxNiyP nanoparticles is tuned finely by tailoring the cationic ratio of Co and Ni. Fe1Co2Ni1P/VrGO exhibits the lowest overpotential (109 and 139 mV) at 10 mA cm−2 and lowest tafel slope (133 and 31 mV dec−1) for hydrogen evolution reaction in 1.0 M KOH and 0.5 M H2SO4, respectively. Fe1Co1Ni2P/VrGO exhibits the lowest overpotential (342 mV) at 10 mA cm−2 with lowest tafel slope (60 mV dec−1) for oxygen evolution reaction. The enhanced performance of the electrocatalyst is attributed to improved electrical conductivity, synergistic effects, and beneficial electronic states caused by the appropriate atomic ratio of Co and Ni in the bifunctional electrocatalyst. This study helps to explore the effect of variable cationic ratio in the cost-effective ternary iron group metal phosphides electrocatalysts to achieve enhanced electrocatalytic performance for water splitting in a wide pH range.