Mineral mixing, a fundamental process during mantle convection, alters the chemical composition of mantle minerals. However, the impact of this process on the electrical conductivity of mineral assemblages remains poorly understood. We measured the electrical conductivity of three single-phase garnets and their corresponding mixtures at 1.5 GPa and varying temperatures using the impedance spectroscopy within frequency from 10⁻¹ to 10⁶ Hz. The electrical conductivity of dehydrated garnets is primarily controlled by their iron content, exhibiting an activation energy about 1.0 eV, indicative of small polaron conduction. The garnet mixture displays lower electrical conductivities and higher activation energies compared to their single-phase counterparts. This discrepancy of conductivity can be half order of magnitude at high temperatures (>1 073 K), suggesting formation of resistive grain boundaries during the mixing process. In the mantle transition zones, grain boundary conductivity could exert a limited impact on the bulk conductivity of the interface between the stagnant slab and ambient mantle.