AbstractAmorphous calcium carbonate (ACC) is a common precursor to crystalline calcium carbonate, and is of particular importance in biomineralization, where its crystallization in privileged environments ensures a pseudomorphic transformation. While organisms regulate this process using organic molecules and magnesium ions to selectively form calcite or aragonite, it has proven highly challenging to replicate this polymorph selectivity synthetically. Here, it is demonstrated that remarkable control can be achieved over the chemical composition and structure of crystalline calcium carbonate by using heat to drive a pseudomorphic transformation of ACC thin films. The crystal polymorph can be tuned from low magnesium‐calcite to pure aragonite, high magnesium‐calcite, and ultimately dolomite according to the magnesium content of the ACC, and mosaics of large single crystals are generated at elevated temperatures rather than the spherulitic structures formed at room temperature. This methodology also enables an in situ investigation of the ACC crystallization mechanism using transmission electron microscopy. Finally, the approach can be combined with templating methods to generate arrays of large aragonite single crystals with preselected morphologies. These results demonstrate that exceptional control can be achieved through the solid‐state transformation of Mg‐ACC, which has relevance to both synthetic and biological systems.