The influence of carbon impurities on phase composition, microstructure, and mechanical properties of hot-pressed ZrB2–SiC ceramics was studied. Carbon was either present in the starting ZrB2 powders or introduced due to the carbon-rich sintering environment. Carbon present in the starting ZrB2 powders removed the inherent oxygen impurities to enable ceramics that appeared phase pure by X-ray diffraction, had smaller average grain sizes, and had higher Vickers’ hardness, but lower fracture toughness. Carbon introduced from the sintering environment led to the formation of secondary ZrC, allowed the microstructure to coarsen, and decreased the fracture toughness. This study showed that ZrB2–SiC ceramics with refined microstructure could be obtained by introducing carbon impurities in the starting ZrB2 powders, whereas the densification should be carried out at lower temperature to avoid the carbon uptake from sintering environment.