ZrC-SiC nanocomposites were synthesized via a facile, green hydrothermal precursor conversion method. The zirconium, silicon and decomposed glucose can be successfully incorporated into a homogeneous framework. In subsequent pyrolysis, the precursor transforms to a core-matrix structure of the nano-ZrO2 and amorphous carbon/silica mixture, resulting in a short diffusion path and limited grain growth. Carbide first appears at a low temperature of 1200 °C, and the conversion is completed at 1500 °C with low oxygen content. The composite consists of grains about 100 nm, exhibiting specific embedded morphology, and has different oxidation resistance in three temperature zones based on the component. The origin mechanism and properties have been elucidated and analyzed. The present work demonstrates the effectiveness of hydrothermal chemistry for the synthesis of carbide composites and their promising application in high temperature protection. Such nanocomposites with controllable morphology and high sinterability beneficial for subsequent densification are also verified. The additive-free ceramic has been obtained with almost fully density (relative density >99%) at a low temperature of 1700 °C by spark plasma sintering (SPS). The sintered specimen possesses fine microstructure with hierarchical grain size distribution (about 1 μm and 100 nm, respectively) and good mechanical properties (fracture toughness of 4.3 ± 0.4 MPa m1/2 and Vickers hardness of 22.8 ± 0.7 GPa).