Abstract: A polymer-derived ZrC ceramic with excellent electromagnetic interference (EMI) shielding performance was developed to meet ultra-high temperature requirements. The thermal decomposition process of ZrC organic precursor was studied to reveal the evolution of phase composition, microstructure, and EMI shielding performance. Furthermore, the carbothermal reduction reaction occurred at 1300°C, and the transition from ZrO₂ to ZrC was completed at 1700°C. With the increase in the annealing temperature, the tetragonal zirconia gradually transformed into monoclinic zirconia, and the transition was completed at the annealing temperature of 1500°C due to the consumption of a large amount of the carbon phase. The average total shielding effectiveness values were 11.63, 22.67, 22.91, 22.81, and 34.73 dB when the polymer-derived ZrC was annealed at 900, 1100, 1300, 1500, and 1700°C, respectively. During the thermal decomposition process, the graphitization degree and phase distribution of free carbon played a dominant role in the shielding performance. The typical core–shell structure composed of carbon and ZrC can be formed at the annealing temperature of 1700°C, which results in excellent shielding performance.