Magnesium alloys are promising ultralight structural materials for aerospace, automotive, and electronic applications owing to their low density, high specific strength, and excellent vibration-damping properties. However, their wider application is restricted by their inadequate corrosion resistance. This study investigates the effect of severe plastic deformation via the KoBo method on the corrosion behavior of Mg–Li alloys containing 0.8 wt.% Ca and 2.0 wt.% rare-earth (RE) additions. Three alloys with lithium contents of 4, 8, and 12 wt.% were examined, emphasizing the relationship between chemical composition, phase evolution (HCP α vs. BCC β), and electrochemical performance in a 3.5 wt.% NaCl solution. Results demonstrate that lithium content plays a decisive role in dictating the corrosion mechanisms. Among the tested materials, the Mg–8Li–0.8Ca–RE alloy exhibited the optimal electrochemical performance, featuring the lowest corrosion current density (i_corr = 0.75 µA/cm²) and most noble corrosion potential (E_corr = -0.959 V vs. SCE), attributed to its ultrafine dual-phase (α+β) microstructure that promotes uniform corrosion and localized oxide film passivation. In contrast, increasing the lithium content to 12 wt.% (single BCC β phase) drastically deteriorated corrosion resistance (i_corr = 334.17 µA/cm^²), resulting in severe pitting and surface destruction. The Mg–4Li-0.8Ca-RE alloy demonstrated intermediate performance (i_corr = 3.49 µA/cm^²). These quantitative findings confirm that precise control of lithium content at 8 wt.%, combined with severe plastic deformation via the KoBo method, effectively optimizes the corrosion resistance of ultralight magnesium alloys.