Electrodeposited multi metallic phosphides represent a versatile platform for HER catalysis. However, the relationships between nucleation behaviour, structural evolution, and activity remain underdeveloped. In this work, FeCoNiP alloy films were prepared at well-controlled cathodic potentials (−1.2 to −1.5 V vs Ag/AgCl) to illustrate how deposition bias dictates phase formation and catalytic performance. Chronoamperometry coupled with Scharifker-Hills modelling reveals a transition from mixed instantaneous-to-progressive nucleation at −1.2 V to instantaneous-dominated 3D nucleation at −1.5 V, reflecting accelerated site activation and rapid surface saturation under strong driving force, while accordingly enhancing island density (7.08 × 107 to 9.10 × 107 cm−2) and nucleation rate (1.69 × 106 → 2.53 × 106 cm−2 s−1). Correspondingly, SEM shows the morphological evolution from coarser grains to compact nanoscale nodular morphologies, while XRD confirms the potential-dependent formation of Ni5P4, Fe2P, and CoP with enhanced defect density at higher bias. Such structural refinements enable outstanding HER activity. The −1.5 V film delivers the lowest overpotential of 134 mV at 10 mA cm−2 and a Tafel slope of 31.7 mV dec−1 in 1 M KOH. Spin-polarized DFT calculations reveal that introducing phosphorus significantly reorganizes the electronic structure of FeCoNi, shifting the d-band center and yielding a nearly ideal hydrogen adsorption free energy (ΔGH* = +0.01 eV) which provides mechanistic insight into the experimentally observed enhancement in HER activity. The present work identifies deposition potential as a decisive lever to control nucleation pathways and optimize catalytic architecture, positioning FeCoNiP as a highly efficient multi metallic phosphide electrocatalyst toward alkaline water electrolysis.