Long-term degradation and failure in high-temperature polymer matrix composites are driven by chemical changes due to oxidation reactions and damage evolution. In this paper, we present a methodology for simulating oxidation-induced damage in a unidirectional composite. This approach explicitly models the time-dependent growth of oxidation layers and the evolution of discrete cracking in a homogenized representation of the composite. Long-term isothermal aging is simulated with high-resolution tracking of morphological changes and damage evolution. An element-free Galerkin method is used to simulate the oxidation layer growth, and the extended finite element method is used for computing the stress fields and predicting damage. The developed model captures both oxidation and damage growth in the unidirectional lamina through long periods of oxidative aging. The model predictions correlate well with the experimental results for a carbon/polyimide composite system.