Mg₂Si is an important parent compound of green thermoelectric materials for medium temperature range applications of power generation. Notwithstanding decades of extensive study on the experimental side, in-depth theoretical understanding of the phonon spectrum and the thermal transport in Mg₂Si from a first-principles perspective was only attained recently. Furthermore, there lacks a feasible first-principles methodology of calculating the electrical transport properties of Mg₂Si. We here present the results of ab initio calculations on the electrical transport properties of Mg₂Si. The microscopic electrical transport parameters such as mobility and mean free path are calculated within the framework of Boltzmann Transport Equation (BTE) in conjunction with ab initio calculations of electron-phonon coupling constants. Both the BTE and the relaxation time are solved iteratively, so empirical or semi-empirical potentials or constant relaxation approximations are no longer needed. The calculated results are in good agreement with experimental reports. The applicability of the present methodology to other thermoelectric materials will be also discussed towards fully calculated figures of merit.