Monohydrocalcite (MHC, CaCO3·H2O) is a thermodynamically metastable phase relative to calcite and aragonite in aqueous solution. Although MHC occurs broadly in organisms, little information about its biogenic origin is available. In this paper, a series of amorphous calcium carbonates (ACCs) with different Mg2+ contents were first synthesized in the presence of polyaspartic acid (PASP), and the phase transformations of the PASP-regulated Mg-ACCs (PASP-Mg-ACCs) were then studied under different medium conditions. The structure, morphology and composition of the precursor PASP-Mg-ACCs and transformation products were investigated by using a wide range of techniques, including XRD, FT-IR, FESEM, TG-DTA, 13C NMR and ICP-AES. Our results show that PASP-Mg-ACC with 24.71 mol% Mg2+ can be transformed into MHC, without formation of hydromagnesite or other hydrated magnesium carbonates, whereas the PASP-Mg-ACCs with <24 mol% Mg2+ resulted in magnesian calcite and aragonite. Time-course transition experiments unveiled that the transformation from PASP-Mg-ACCs to crystalline phases proceeds through the dissolution of the initial precursor PASP-Mg-ACCs and the subsequent crystallization of the secondary mineral phases. The formation of different secondary minerals depends not only on the Mg2+ but also on the PASP content in precursor PASP-Mg-ACCs. Because of the exclusive formation of MHC from PASP-Mg-ACC under current biomimetic conditions, biomacromolecule-regulated Mg-ACC may act as the transient precursor and be responsible for the biogenesis of MHC. The current results contribute to the elucidation of the biogenic origin and role of MHC in nature.