Graphene oxide (GO) and reduced graphene oxide exhibit proton and electron (or hole) conduction, respectively. Owing to this, the conductivity of GO can be controlled via reduction because its electron conductivity increases and its proton conductivity depends on the concentration of epoxide groups. Herein, we report the successful control of the proton and electron conductivities of GO using the photoirradiation and thermal reduction processes. The proton conductivity decreases when the epoxide content and layer distance decreases, whereas the electron conductivity drastically increases with decreasing oxygen content. Both the electron and proton conduction mechanisms for GO are discussed based on the concentrations of various functional groups and defects, changes in the interlayer distance, and the activation energy associated with proton conduction. Finally, we determined the most suitable degree of reduction for obtaining a good mixed conductor useful as an electrode material and a hydrogen separation membrane that functions at room temperature.