Impact of insertion of a single Fe, Co, Ni or Cu atom into crystalline lattice of orthorhombic Mo2C(001) surface on fundamental properties of the surface atoms has been investigated by means of density functional theory. Polar C- and Mo-terminated surfaces were used as model systems and two modes of the foreign atom insertion were considered: into the topmost Mo layer of the surface and into metal layer deeper in the structure of the host system. Calculations revealed that TM-modified Mo2C compounds were less stable than the pristine carbide with no significant changes in work function of the carbide surface, regardless of its termination, foreign atom nature, and its position in the host system. Furthermore, presence of either Fe, Co, Ni or Cu atom in subsurface or in the topmost metal layer of the terminations did not impact noticeably electronic structure of surface C and Mo atoms closest to the modification site as evidenced from analysis of their density of states. Consequentially, insertion of foreign atoms into the lattice of molybdenum carbide results in atomic hydrogen stability close to that on the corresponding unmodified polar termination of the Mo2C(001) surface. In the context of experimentally observed changes in catalytic activity of molybdenum carbide toward hydrogen evolution reaction upon doping it with transition metals, the obtained data evidence that the major effect comes from the foreign atoms being adsorbed on molybdenum carbide surface, while insertion into the lattice bears no significant impact. This suggests that coating Mo2C surface with transition metals can be promising strategy to modify and tailor its catalytic properties for hydrogen production. © 2024