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 calcium and rare-earth (RE) additions. Three alloys with lithium contents of 4, 8, and 12 wt.%, were examined emphasizing the relationship between chemical composition, microstructural evolution, and electrochemical performance in a chloride-containing environment. Results demonstrate that lithium content plays a decisive role in determining the corrosion resistance of the investigated alloys. Among the tested materials, the Mg–8Li–0.8Ca–RE alloy exhibited the most favorable electrochemical characteristics, attributed to its refined dual-phase microstructure that promotes more uniform corrosion and partial surface passivation. In contrast, increasing the lithium content to 12 wt.% significantly deteriorated corrosion resistance, resulting in intensified surface degradation and localized pitting corrosion. These findings confirm that appropriate control of lithium content combined with advanced severe plastic deformation by the KoBo method effectively enhances the corrosion performance of ultralight magnesium alloys, demonstrating their considerable potential for applications requiring both low weight and durability in aggressive service environments.