The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at temperatures in the 274–316 °C range. The experimental program was developed to explore the influence of temperature on strain-controlled tensile loading, relaxation and creep behaviors. The experimental results clearly demonstrate that the mechanical behavior of PMR-15 polymer exhibits a strong dependence on temperature. During strain-controlled tensile loading, the slope of the stress-strain curve in the quasi-elastic region decreases and the slope of the stress-strain curve in the flow stress region increases with increasing temperature. At a given strain rate, the flow stress level decreases with increasing temperature. Furthermore, the transition from quasi-elastic behavior to inelastic flow becomes less pronounced with increasing temperature. During relaxation, the amount of the stress drop for a given prior strain rate decreases with increasing temperature. At a given prior strain rate and creep stress level, increasing temperature results in increased creep strain accumulation. Based on the experimental results the Viscoplasticity Based on Overstress for Polymers (VBOP) theory was augmented to account for the effects of elevated temperature. Several model parameters were determined to depend on temperature. Those parameters were developed into functions of temperature. The augmented VBOP was then employed to predict the response of the PMR-15 polymer under various test histories at temperatures in the 274–316 °C range. An enhanced procedure for determining VBOP model parameters that utilizes a McLean type dip test to assess the equilibrium stress was developed. Model predictions were considerably improved by employing an enhanced model characterization procedure. Additionally, the effects of prior isothermal aging at various temperatures in the 260–316 °C range on the inelastic deformation behavior of PMR-15 at 288 °C were evaluated. For PMR-15 aged at 260–302 °C, the initial slope of the stress-strain curve and the flow stress increase with prior aging duration. The shape of the knee of the stress-strain curve becomes more pronounced and the departure from quasi-linear behavior is delayed with increasing prior aging time. Experimental results reveal that there is no relationship between prior aging temperature and elastic modulus, tangent modulus, the shape of the knee of the stress-strain curve or departure from quasi-linear behavior. This implies that for aging temperatures in the 260–302 °C range, mechanical behavior depends only on prior aging duration and not on prior aging temperature. However, testing of PMR-15 aged at 316 °C revealed a decrease in flow stress with increasing prior aging duration. This suggests that a degradation mechanism, not present at lower aging temperatures, is in effect during aging at 316 °C.