A stoichiometric MgFePO4F (MFPF) is synthesised by using a solid-state carbothermal method. Its monoclinic framework, possessing an entire cationic mixing of Mg2+ and Fe2+, is validated via both crystal structure analysis and simulation. Interestingly, MFPF exhibits a relatively high potential (2.6 V vs. Mg/Mg2+) and good cyclic stability with an encouraging capacity (53 mA h g−1), bringing MFPF to the fore as a promising cathode material for magnesium batteries.