The mimicry of bio-membrane with a liquid/liquid interface between two immiscible electrolyte solutions is intrinsically defect-free to study catalysis of energy conversion reactions i.e., CO2 reduction, oxygen reduction, and hydrogen evolution. Herein, we report the in-situ generation of electrodeposited black phosphorous (BP) based nanocomposites at the liquid/liquid interface for the first time and their catalysis in hydrogen evolution reaction (HER). The catalytic HER activities of these catalysts have been investigated electrochemically and also chemically by two-phase reactions. The BP/MoSx, BP/Cu, and BP/Pt nanocomposites were formed by reducing the catalyst precursors such as (NH4)2MoS4, (NH4)2PtCl4, and CuCl2 salts, respectively on the BP nanosheets by decamethylferrocene (DMFc) electron donor during the catalytic HER. The electrodeposited nanocomposites were collected from the interface and characterized by using advanced analytical techniques. Among them, the BP/MoSx nanocomposites showed the highest HER activity with a reaction rate constant of 0.202 min−1 was about 230- and 7-times greater than the ones obtained by non-catalytic reaction and the free-MoSx catalyst. Moreover, the nucleation of the catalysts and the HER mechanisms were also explained in detail. The BP/MoSx also showed higher HER activity compared to that of carbon nanotubes CNTMoSx and reduced graphene oxide rGOMoSx nanocomposites.