Negatively charged silicon vacancy (VSi−) defects in silicon carbide are expected to be used for magnetic sensors under harsh environments, such as space and underground due to their structural stability and potential for high-fidelity spin manipulation at high temperatures. To realize VSi− based magnetic sensors operating at high temperatures, the temperature dependence of optically detected magnetic resonance (ODMR) in the ground states of VSi− defects, which is the basic principle of magnetic sensing, should be systematically understood. In this work, we demonstrate the potential of VSi− magnetic sensors up to at least 591 K by showing the ODMR spectra with different temperatures. Furthermore, the resonance frequency of the ground level was independent of temperature, indicating the potential for calibration-free magnetic sensors in temperature-varying environments. We also characterize the concentration of VSi− defects formed by electron irradiation and clarify the relationship of magnetic sensing sensitivity to VSi− concentration and find that the sensing sensitivity increases linearly with VSi− concentration up to at least 6.0 × 1016 cm−3. The magnetic sensitivity at a temperature above 549 K was reduced by half as compared to that at 300 K. The results pave the way for the use of a highly sensitive VSi−-based magnetic sensor under harsh environments.