We investigated the effect of Mg2+ on calcite hillock growth over a broad range of solution conditions in terms of supersaturation (Ωcalcite) and Mg/Ca ratios using atomic force microscopy and secondary ion mass spectrometry. We found that both the incorporation pattern/incorporated Mg2+ quantity in the hillock structure and the Mg2+-induced morphological change of the hillock surface showed strong dependence of the growth conditions. Specifically, when Mg/Ca was high (i.e., >5) and Ωcalcite was low (i.e., ~0.45), Mg2+ was predominantly incorporated into the negative sectors of the hillock structure, resulting in gradual loss of step structure and morphological amorphism on these vicinal surfaces. When Mg/Ca and Ωcalcite were in intermediate ranges (i.e., Mg/Ca < 5, and 0.45 < Ωcalcite < 1), the originally straight edges of the hillock steps exhibited curvatures of varying degrees and formed “tear-drop” morphologies. It is noted that such “tear-drop” morphology was stable within the duration of the experiments and did not evolve into other surface patterns. By contrast, when both Mg/Ca and Ωcalcite were high (i.e., Mg/Ca > 5, and Ωcalcite > 1.1), the growing hillocks experienced two phases of morphological changes, initiated with the formation of “tear-drops” followed by the development of linear ruptures along [4̄81] and [441̄] directions. And the occurrence of these ruptures segmented the hillock surface effectively into multiple isolated plateaus. Significantly, we revealed the underlying mechanisms for these condition-specific effects of Mg2+ on calcite growth, which mainly resulted from the interplay among three major factors: (1) the size-mismatch between Mg2+ and Ca2+ that causes structural strains in magnesian calcite and leads to morphological amorphism in high-Mg carbonate; (2) the asymmetry of the calcite crystal structure that sets a physical limitation for Mg2+ incorporation patterns in the hillock structure; and (3) the step advancing rate (i.e., the calcite growth kinetics) that affects both Mg2+ incorporation and the accommodation of Mg2+-induced structural strains in the hillock structure. Detailed discussions were given for each growth scenario. The results of our study provide a theoretical base to decipher the roles of Mg2+ in CaCO3 mineralization, and thus, have important implication for a range of processes that involve the growth of Mg-Ca-CO3 systems, such as biomineralization, carbon capture and storage, and scale controls in industrial settings.