Under ambient conditions, marine organisms are able to synthesize a variety of functional materials, ranging from eye lenses to protective shells through the meticulous control over magnesium incorporation into calcite during its crystallization. The mechanistic understanding of how they achieve such exquisite control, at a constant magnesium-to-calcium ratio and at ambient conditions, is important in the development of bioinspired functional materials. However, the replication of these processes in the laboratory is still challenging. Herein, we present a systematic study on how to tune magnesium incorporation into calcite and polymorph selection in the Ca-Mg-CO3 system through the precise control of the inorganic solutions chemistry at ambient conditions of temperature and pressure, and at a magnesium-to-calcium ratio of 5:1, which is analogous to the ratio found in most seas. By varying the pH, cation-to-anion ratio, and solution concentration, the controlled synthesis of magnesium calcites with 10-45% magnesium was achieved at room temperature. The mechanism of formation is consistent with that observed during biomineralization, during which an intermediate magnesium-rich amorphous calcium carbonate (Mg-ACC) phase forms first and later transforms into high magnesium calcite. Once crystallization occurs, the magnesium calcites that form are stable in solution and exhibit slow growth through Ostwald ripening. Our findings suggest that the precise control of saturation levels is key in driving nucleation and crystallization.