Aqueous magnesium ion supercapacitors (MISs) have attracted attention due to their safety, low cost and environmental friendliness. However, the cycling stability of MISs is usually not ideal due to magnesium ion plating in/stripping from the negative electrode materials. Here, we demonstrate that MoS₂ with expanded interlayer spacing (E-MoS₂), obtained via a facile method, is a prospective negative electrode material for rechargeable MISs, because the expanded layer spacing reduces ion diffusion resistance and provides more active sites for ion interaction. The specific capacitance of the E-MoS₂ electrode (165.6 F g⁻¹) is much higher than that of unmodified MoS₂ (79.0 F g⁻¹) at 0.5 A g⁻¹. Importantly, after 30 000 cycles, the E-MoS₂-based MIS has an ultralong cycling life with a capacitance retention of 93.8% at 5 A g⁻¹, which is comparable with conventional solid-state capacitors. This strategy provides an effective method to design high-performance electrode material for MISs.