The performance of a primary Mg-air cell was evaluated at room temperature using a 72mol% ethylene glycol/trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]) ionic liquid (IL) electrolyte. The cell was cycling in ambient air as well as in the presence of pure oxygen, and interestingly the cell presented much higher discharge capacity in air than in oxygen, which was attributed to the effect of water in the ambient air. When operated in ambient air, the cell showed promising discharge behaviour with a maximum rate of 0.2mAcm−2 and a discharge capacity of around 4.8mAhcm−2. When operated at a low rate 0.0075mAcm−2, the cell lasted for over 260h, 10 days, at a potential above 1.3V. Thus, the main focus of this study is the analysis of the mechanism of discharge capacity loss in this electrolyte, which revealed that, both the polarization due to the presence of a resistive Mg interphase on the anode surface and, concentration polarization due to the quick accumulation of Mg2+ ions in the IL based electrolyte are responsible. In-depth surface characterization suggested the discharge products accumulated on the Mg surface with a proposed formula [P6,6,6,14]·Cl·Mg(OH)2·9[Mg(OCH2CH2OH)Cl]·40H2O most likely had a highly-crosslinked chemical structure, which were responsible for the limited ionic conductivity of the Mg interphase.