•Phase-separated NiCuMn was synthesized via one-step electrochemical deposition.•Multicomponent heterostructures synergistically accelerate alkaline HER kinetics.•NiCuMn possesses overpotential of 17 and 62 mV to deliver 10 and 100 mA cm−2. Constructing catalytic active sites to synergistically promote both Volmer and Heyrovsky/Tafel steps and understand the underlying mechanism are of paramount importance for designing the alkaline hydrogen evolution reaction (HER). Herein, a facile one-step electro-deposition strategy is developed to synthesize a novel phase-separated NiCu/NiMn(OH)2 nanocomposite (NiCuMn). Theoretical and experimental results demonstrate that low electronegativity Mn trigger the formation of low valence state Niδ+ in NiMn(OH)2 to accelerate the Volmer step while Cu can optimize the d-band center of NiCu alloy to facilitate the Heyrovsky/Tafel step. Benefiting from this steering of elementary steps by the synergistic components, the obtained NiCuMn nanocomposite achieves ultralow overpotential of 17 and 62 mV to drive 10 and 100 mA cm−2 in 1 M KOH, respectively, and ultrahigh turnover frequency of 0.78 s−1 at overpotential of 100 mV. Moreover, NiCuMn exhibits outstanding durability with a slight potential degradation after 10,000 cyclic voltammetric test. This study not only provides atomic-level insight into optimizing the elementary steps through dual sites synergies, but also introduces a new way to simply prepare advanced HER electrocatalysts and beyond.