Amorphous calcium carbonate (ACC) serves as a precursor to crystalline biominerals. Both inorganic and organic additives dramatically impact its formation and transformation, but these processes have largely been the subject of bulk assays while direct observations are lacking. Here we report in situ TEM experiments on the effects of Mg, citrate and poly-acrylate (PAA) on the ACC-to-crystal transformation. For citrate, PAA and ≤ 2.5 mM Mg, we find dissolution/re-precipitation dominates, with each additive extending the ACC lifetime and/or timescale for dissolution, but the final crystals exhibit expected morphologies. In contrast, for Mg ≥ 5 mM, transformation occurs in the absence of a morphological change to give spheroidal Mg-calcite and is accompanied by loss of structural water seen both via an increase in electron density and changes in Raman spectra. TGA shows Mg brings excess water into ACC and molecular dynamics simulations predict this promotes atomic rearrangement. We hypothesize that slow dehydration coupled with ease of reorganization enables the isomorphic conversion and may explain why Mg is a common impurity in biominerals, as its presence would allow ACC to be moulded into complex shapes and maintain those shapes while transforming into a crystalline polymorph.