Cast steel joints are widely used in engineering structures and exhibit different mechanical responses in service environments at elevated temperatures. Hence, the temperature-dependent mechanical properties in cast steel joint must be characterized to guide the engineering design and service life assessment at elevated temperatures. This study carried out a comprehensive, experimental investigation of the base materials of cast steel ZG300–500 H and vessel steel Q345R cut from a cast steel joint to obtain the reduction factors and stress–strain responses at elevated temperatures. The reduction factors of yield strength, elastic modulus, and ultimate strength were analyzed and compared with the code-recommended values and obtained test data. The material properties of ZG300–500 H and Q345R deteriorated significantly as the temperature increased, and the currently available reduction factors in the design codes and obtained test data were incapable of accurately describing their material performance degradation. Therefore, new predictive equations have been proposed to accurately describe the mechanical properties’ degradation and stress–strain responses of ZG300–500 H and Q345R at elevated temperatures of 20–700 °C. These research results also provide a strong base for the safety research and design of cast steel joint structures at elevated temperatures.