Metal dichalcogenide based 2D materials, gained considerable attention recently as a hydrogen evolution reaction (HER) electrocatalyst. In this work, we synthesized MoSe2 based electrocatalyst via hydrothermal route with varying phase contents (1T/2H) and respective HER performances were evaluated under the acidic media (0.5M H2SO4), where best HER performance was obtained from the sample consisting of mixed 1T/2H phases, which was directly grown on a carbon paper (167mV at 10mA/cm2) Furthermore, HER performance of electrocatalyst was further improved by in-situ electrodeposition of Pt nanoparticles (0.15 wt%) on the MoSe2 surface, which lead to significant enhancement in the HER performances (133mV at 10mA/cm2). Finally, we conducted DFT calculations to reveal the origin of such enhanced performances when the mixed 1T/2H phases were present, where phase boundary region (1T/2H heterojunction) act as a low energy pathway for H2 adsorption and desorption via electron accumulation effect. Moreover, presence of the Pt nanoparticles tunes the electronic states of the MoSe2 based catalyst, resulting in the enhanced HER activity at heterointerface of 1T/2H MoSe2 while facilitating the hydrogen adsorption and desorption process providing a low energy pathway for HER. These results provide new insight on atomic level understanding of the MoSe2 based catalyst for HER application.