This paper describes mechanism-based modeling of damage evolution in high temperature polymer matrix composites (HTPMC) under thermo-oxidative aging conditions. Specifically, a multi-scale model based on micro-mechanics analysis in conjunction with continuum damage mechanics (CDM) is developed to simulate the accelerated fiber-matrix debond growth in the longitudinal direction of a unidirectional HTPMC. Using this approach, one can relate the behavior of composites at the micro-level (representative volume element) to the macro-level (structural element) in a computationally tractable manner. Thermo-oxidative aging is simulated with diffusion-reaction model in which temperature, oxygen concentration, and weight loss effects are considered. For debond growth simulation, a model based on Darcy's laws for oxygen permeation in the fiber-matrix interface is employed, that, when coupled with polymer shrinkage, provides a mechanism for permeation-controlled debond growth in HTPMC. Benchmark of model prediction with experimental observations of oxidation layer growth is presented, together with a laminate thermo-oxidative life prediction model based on CDM to demonstrate proof-of-concept.