This paper presents a physics-based constitutive equation to predict the mechanical responses of thermochemically aged elastomers. High-temperature oxidation in elastomers is a complex phenomenon. The macromolecular network of elastomers’ microstructures undergoes chain scission and crosslinking under high temperature and oxygen saturation conditions. In this work, we modify the network stiffness and the chain extensibility in the well-known Arruda–Boyce constitutive equation to incorporate network changes in the microstructures of elastomers during thermochemical aging. In particular, the effects of network evolution due to aging in changing the shear modulus and the number of Kuhn monomers are considered. The modification is based on chemical characterization tests measuring the crosslink density evolution. The developed constitutive equation predicts the mechanical responses of thermochemically aged elastomers independently of any mechanical tests on aged samples. The proposed constitutive equation is validated with respect to a comprehensive set of experimental data available in the literature that were designed to capture thermochemical aging effects in elastomers. The comparison showed that the developed constitutive equation can accurately predict the tensile tests conducted on aged samples based on crosslink density evolution input. The obtained constitutive equation is physics-based, simple, and includes minimal material parameters.