In this work we elucidate real-time mechanistic insights into the promotional effect of Ni substitution on the bifunctional overall water splitting (OWS) activity of molybdenum and tungsten carbides (Ni-MoC/WC@NGC) supported on N-doped graphitic carbon (NGC). Ni substitution yields multi-fold improvement in OWS over the pristine systems that are comparable to commercial Pt/C for hydrogen evolution reaction (HER) and better than IrO2 for oxygen evolution reaction (OER). Ni-MoC@NGC champions in HER activity exhibiting an onset overpotential of 65 mV and current density of 140 mA/cm2 at −370 mV (v RHE) in acidic media. An H2O electrolyzer constructed with Ni-MoC@NGC shows a comparable cell voltage to the PtǁIrO2 pair and could split water aided by 1.5 V AAA commercial battery. First principle calculations and in-situ probing through quick-XAS during electrochemical processes provide valuable insights into how the adsorption energies of intermediates and reaction kinetics are modulated at different catalytic sites with the promotional electronic effect of Ni.