In situ deposited catalysts are drawing great attention in the hydrogen evolution reaction for photocatalytic and electrocatalytic processes due to their inexpensive and simple preparation methods. Molybdenum sulfide derivatives are convenient alternatives to the well-known and efficient noble metallic catalyst Pt due to their uncostly and abundant nature. Herein, liquid/liquid interfaces are chosen to determine the catalytic activity of a template-free nanocomposite catalyst composed of MoSx grown in situ on multiwalled CNTs (CNT/MoSx) during catalytic hydrogen production for the first time. The organic sacrificial agent decamethylferrocene plays the role of a reductant for both (NH4)2MoS4 and protons to obtain MoSx and molecular hydrogen, respectively. The catalytic activity of CNT/MoSx is investigated by four-electrode voltammetry and biphasic reactions at the water/1,2-dichloroethane (DCE) interface. In addition, the in situ obtained CNT/MoSx nanocomposite catalyst is isolated from the interface and characterized by morphological and structural techniques. Moreover, the reaction kinetics for hydrogen production is calculated by real-time UV-vis absorption spectroscopy via measuring decamethylferrocenium concentrations. The hydrogen evolution reaction rate of CNT/MoSx increases by 85- and 2.5-fold compared with those of the uncatalyzed reaction and free-MoSx, respectively. The increased catalytic activity of CNT/MoSx is based on the enhanced charge transport efficiency of CNTs due to their one-dimensional (1D) structure, high electrical conductivity, excess active sites on MoSx, and the synergetic effect between CNTs and MoSx. This study paves the way for preparing nanocomposite catalysts with different substrates and also different energy applications using the CNT/MoSx nanocomposite catalyst.