Metastable γ-MnS and stable α-MnS were synthesized via a one-step facile hydrothermal method and investigated as anode materials in Li-ion batteries and capacitors for revealing the essence of their electrochemical difference. Comparing with octahedrally coordinated [MnS6] in stable α-MnS, the Mn in γ-MnS has a tetrahedral coordination with smaller crystal field splitting and the Mn–S bond with a stronger covalent character, resulting in a lower discharge potential that enables assembled devices with a higher energy density. Density functional theory (DFT) calculation corroborates that the metastable γ-MnS possesses high charge density overlapping in the chemical bonds and decreased energy barriers for the reactive electrons deviating from 3d orbitals. Metastable γ-MnS delivers a larger storage capacity of 705 mAh g−1 at a current density of 100 mA g−1, and also demonstrates superior rate capability (362 mAh g−1 at 2 A g−1) and cycling stability (550 mAh g−1 remaining after 200 cycles at 0.5 A g−1) in Li-ion half cells. Moreover, the γ-MnS//AC hybrid capacitor with a wide voltage window of 0.1–4 V demonstrates a maximum energy and power density of 220 W h kg−1 and 10133 W kg−1 respectively, and cycles at 5 A g−1 without detectable capacity fading over 5000 cycles.