Amorphous calcium carbonate (ACC) plays a crucial role in biomineralization which crystallization process has attracted significant attention. Magnesium ions (Mg²⁺) can effectively regulate the crystallization of ACC, but the mechanism by which it controls the transformation of ACC into monohydrocalcite (MHC, CaCO₃·H₂O) is not well understood. In this study, Mg²⁺ was used as an additive, and the transformation process from ACC to MHC was investigated in situ using an automatic potentiometric titration system. It was found that Mg²⁺ can enhance the stability of ACC and inhibit the formation of calcite and vaterite. During the transformation of ACC to MHC, partial dissolution firstly occurred, and the molar ratio of Mg/Ca in the solution increased with the consumption of Ca²⁺. Mg²⁺ further adsorbed onto the surface of ACC particles, inhibiting surface dissolution of ACC and promoting internal dissolution of ACC, resulting in the formation of hollow structures rich in Mg²⁺ and smaller-sized nanoparticles. Subsequently, MHC crystallized and grew through particle aggregation. These results elucidate the mechanism by which Mg²⁺ regulates the transformation of ACC into MHC through a non-classical crystallization pathway, enhancing an understanding of the biomineralization mechanism from ACC precursor.