The physical and chemical properties of atomically thin two-dimensional (2D) materials can be modified by the substrates. In this study, the substrate effect on the electrocatalytic hydrogen evolution reaction (HER) in 2D Mo2C monolayers was investigated using first principles calculations. The isolated Mo2C monolayer shows large variation in HER activity depending on hydrogen coverage: it has relatively low activity at low hydrogen coverage but high activity at high hydrogen coverage. Ag, Au, Cu, and graphene were used as substrates to study the substrate effect. While the effects of the Au and graphene substrates on the HER activity are insignificant, Ag and Cu substrates improve the HRE activity, especially at low hydrogen coverage, by modifying the valence electrons in the Mo2C layer; therefore, the HER activity of the Mo2C monolayer becomes high for any hydrogen coverage. Our results suggest that, in two-dimensional electrocatalysis, the substrate has a degree of freedom to tune the catalytic activity.