The benchmark magnesium electrolyte, [Mg₂Cl₃]⁺ [AlPh₄]⁻, can be prepared in a 100% atom-economic fashion by a ligand exchange reaction between AlCl₃ and two molar equivalents of MgPh₂. NMR and vibrational spectroscopy indicate that the reported approach results in a simpler ionic composition than the more widely adopted synthesis route of combining PhMgCl with AlCl₃. Electrochemical performance has been validated by polarisation tests and cyclic voltammetry, which demonstrate excellent stability of electrolytes produced via this atom-efficient approach.