Rechargeable Mg batteries are promising candidates for highly safe, large-scale energy storage batteries due to the low-cost and non-dendritic metallic Mg anode. However, exploring high-performance cathodes remains a great challenge blocking their development. Herein, a rechargeable Mg battery is established with a Ag₂S conversion cathode, providing a highly reversible capacity of 120 mA h g⁻¹ at 50 mA g⁻¹, a superior rate capability of 70 mA h g⁻¹ at 500 mA g⁻¹, and an outstanding long-term cyclability over 400 cycles. The mechanism was investigated using XRD, TEM and XPS in addition to electrochemical measurements, and indicated a two-stage magnesiation: first, Mg²⁺ intercalation into Ag₂S and then a conversion reaction to form metallic Ag⁰ and MgS. The solid-state Mg²⁺ diffusion coefficients are as high as 3.6 × 10⁻⁹ and 3.1 × 10⁻¹⁰ cm² s⁻¹ for the intercalation and conversion reactions, respectively, which explains the high performance of the Ag₂S cathode. This work provides scientific insights for the selection of a promising conversion cathode by the combination of soft anions and soft transition metal cations.