The structural and electronic properties of MnO₂ based electrocatalysts are key factors determining their electrochemical performance. To date, it is still challenging to synergistically tune the crystal structure, morphology, and electronic band (i.e., band gap and band alignments) of MnO₂ through facile synthesis approaches. This study has reported a one-step hydrothermal method to synthesize a prototypical MnO₂ electrocatalyst with optimized structural and electrochemical properties. By simply adjusting the hydrothermal time, the phase transition from polymorphic δ to α can be induced in MnO₂. The obtained nanowires on nanosheet structure grown in-situ on nickel foam provides a large surface area, great accessible active sites, and good mass/charge transfer efficiency. Further investigation through first-principles calculations reveals that compared to δ-MnO_2, the α-MnO₂ polymorph with rich oxygen vacancies has better band-alignment tunability, which is also beneficial for improving the electrochemical performance. The α phase MnO₂ exhibits superior catalytic performance for both OER and HER (OER overpotential of 0.45 V at 50 mA cm^–2 and HER overpotential of 0.14 V at 50 mA cm^–2). The developed synthesis method can be extended to catalyst designs that require precise control of phase and morphology evolution in a wide range of applications.