An exact thermal degradation model for lubricant grease emerges as the sum of the contributions from the active thermal mechanisms of heat transfer, heat storage and evaporation. The second law's non-negative entropy generation is used as the basis for degradation characterization. The fundamental thermal entropy generation relation for the oven aging of grease is correlated with a degradation measure, as prescribed by the Degradation-Entropy Generation theorem. Five grease properties are selected as degradation measures. The model is evaluated by substituting temperature profiles and grease properties measured during oven aging of three different greases. A fit of R2 ≈ 1 was obtained for all the data, notwithstanding the non-monotonic aging evolution of the measured grease properties. Degradation coefficients, which indicate the influence of each active thermal mechanism on grease degradation, are quantified and compared for the three greases. The model is used to rank the greases according to their resistances to thermal degradation. Results show that the lithium-thickened grease with mineral base oil is the most susceptible to thermal degradation, followed by the lithium-thickened grease having a semi-synthetic base oil (a highly synthesized mineral oil with properties similar to synthetic polyalphaolefin oil). The polyurea-thickened grease with fully synthetic oil exhibits the highest thermal degradation resistance. These model predictions corroborate known empirical behaviors of these grease types. The proposed thermal degradation model applies to all greases and any material that undergoes thermal degradation.