Micro-nano-assembled manganese carbonate (MnCO3) was synthesized via aqueous precipitation assisted by ethanol. The effects of time and the flow rate of ethanol introduction on the morphology and size of products were carefully investigated. The results reveal that ethanol may participate in the precipitation of MnCO3 nanoparticles, the retarding of aggregate fusion, and the reassembly of aggregates at different stages. By a delayed but swift introduction of ethanol, we can regulate these effects to endow the micro-nano-assembled MnCO3 with high surface area, uniform morphology, and calcination-resistant skeleton simultaneously. The synthesis of monodispersed micro-nano-assembled MnCO3 at elevated concentrations of reactants is also possible by introducing ethanol earlier.