Understanding the degradation of material properties and stress–strain behavior of rubberlike materials that have been exposed to elevated temperature is essential for rubber components design and life time prediction. The complexity of the relationship between hyperelastic materials, crosslinking density (CLD), and chemical composition presents a difficult problem for the accurate prediction of mechanical properties under thermal aging. In this paper, a new and relatively simple mathematical formulation is presented to expresses the change in material properties of hyperelastic materials under thermal aging. The proposed formulation has been applied to a natural rubber (NR). Testing was performed on more than 130 specimens that were thermally aged then subjected uniaxial tension and hardness tests. The aging temperatures ranged from 76.7 °C to 115.5 °C, and the aging times ranged from 0 to 600 h. Based on the recorded experimental data, the NR mechanical properties under thermal aging showed a similar behavior to the rate of change of the CLD with aging time and temperature. Three mechanical properties have been chosen to be studied in this paper: the ultimate tensile strength, the fracture stretch value, and the secant modulus at 11.0% strain. The proposed mathematical formulation is a phenomenological equation that relates the material properties with the change in CLD based on a form of Arrhenius equation. The proposed equation showed promising results compared to the experimental data with an acceptable error margin of less than 10% in most of the cases studied.