A binary ionic liquid (IL) system based on a common organic cation, N,N-diethyl-N-methyl-N-(2-methoxyethyl)anmmonium (DEME+), with mixed anion of bis(trifluoromethanesulfonyl)imide (TFSI−) and bis(fluorosulfonyl)imide (FSI−) has been examined as a supporting electrolyte of rechargeable magnesium batteries. Basic properties of the mixed IL were measured as a function of the mixing ratio, n, in the composition of [DEME+][TFSI−] n [FSI−]1−n . Electrochemical deposition/dissolution behavior of magnesium was investigated in the mixed IL containing an organo-magnesium complex, methyl magnesium bromide with tetrahydrofuran (MeMgBr/THF). Viscosity and ionic conductivity of the mixed IL showed monotonous changes with the mixing ratio, while their thermal properties proved discontinuous changes with the compositions. Coulombic (current) efficiency of 90% or higher was obtained for cathodic deposition of magnesium in the mixed IL containing MeMgBr/THF. The current density for cathodic deposition and anodic dissolution depended on the IL composition, and the maximum value was obtained at the composition of n =0.5.