Metallic 1Tphase of MoS₂ is desirable for accelerating electrochemical hydrogen generation from water due to its excellent electrical conductivity. However, it is challenging to synthesize stable 1T-MoS₂ on a large scale using conventional chemical or physical routes. Here in, we report highly durable Mn-doping assisted in-plane 1T/2H mixed phases of MoS₂ synthesized by a simple one-step hydrothermal method. The Mn-doping not only results in a favourable electronic modification in the host lattice but also in a structural evolution from 2H (trigonal prismatic) to 1T (octahedral) phase through sulfur-plane gliding with additional defects. The 1T/2H interfaces exhibit more active sites for hydrogen evolution reaction (HER) than its bulk 2H-counterpart. The developed electrode shows relatively low overpotential of 95 mV at a current density of 10 mA/cm² and a low Tafel slope of 72 mV/dec in 0.5 M H₂SO₄. In addition, Density Functional Theory based calculations also support that Mn- doping in pristine 2H-MoS₂ encourages a structural phase separation. Locally hybridized d-orbitals of Mn induce the formation of 2H/1T interfaces. It results in the delocalization of electrons, and thus facilitates the hydrogen adsorption and desorption process, better than the 2H phase.