Low-temperature sintering is a highly effective method for solidifying powders of thermally degradable materials, such as carbonates and hydroxides. Hydrothermal hot-pressing (HHP) is a low-temperature sintering method that uses a dissolution-precipitation reaction to water under hydrothermal conditions. Most sintering techniques, including HHP, were devised to fabricate high-density compacts; hence, varying the internal microstructure during solidification is challenging. The present study focuses on the microstructural development of MnCO3 microsphere compacts through HHP. Depending on the HHP process temperature, the formation of MnOOH fibers and Mn3O4 nanoparticles within the MnCO3 compacts or hollowing of the microspheres during the compaction was observed. The direct production of microstructure-controlled compacts by HHP will contribute to new structural designs and add functionalities to existing applications.