Magnesium metal is potential candidate for high strength to weight ratio alloys with wide application in aerospace and automotive industries. However, poor corrosion resistance under ambient environmental conditions is the bottleneck for industrial deployment. Designing passivation layers and/or corrosion resistance alloys require fundamental understanding of the corrosion process. The traditional ex-situ spectroscopic measurements of polycrystalline metal surface with ubiquitous surface impurities provided indistinct view of the corrosion process. To clearly distinguish the mechanism and sequence of corrosion process, we employed in-situ cryo-based x-ray photoelectron spectroscopy (XPS) measurements on well-defined pure and aqueous solution exposed Mg-single crystal surfaces in combination with ab initio atomistic modelling studies. Clean Mg (0001) surfaces were carefully exposed to pure and sodium chloride mixed water and the subsequent interfacial reactions were studies through integrated experimental and theoretical approach. This study provides atomistic view of magnesium hydroxide nucleation as main product of the corrosion process. Under salt conditions, the competitive nucleation process between magnesium hydroxide and magnesium chloride were observed along with the formation of magnesium chloride hydrate and magnesium hydroxide hydrate phases. By combining the energy requirements from computational modelling and the electronic states of corrosion products, the mechanism and sequence of corrosion process on Mg metal will be discussed.