Fluoroelastomer compositions are used in extreme environments, such as high pressure and high temperature (HPHT), in a wide range of applications in the oil and gas industry. As hydrocarbon production is expanded to deeper formations, and operating temperature and pressure become higher, the material performance of these elastomers becomes more important. Hence, fluoroelastomers face new challenges and require further understanding when these undergo aging. In this study, a practical fluoroelastomer (FKM) composition was aged in deionized water and an alkaline hydraulic fluid up to 31 MPa, 225 °C and 168 hrs. High pressure, high temperature and aging time have deleterious effects on the physical and mechanical properties of FKMs in both the fluids. Spectroscopically, under these conditions, two hydrocarbon-related bands at 2917 and 2850 cm−1 and three fluorocarbon bands at 886, 1158 and 1396 cm−1 in the Fourier Transform Infrared spectra decreased in intensity after aging. The temperature, at which the rate of degradation was maximum, measured through thermogravimetry also decreased under HPHT conditions. It was further observed that the filler particles bound to the rubber leached out during the aging experiment to cause a color change of the fluid. The tensile sheet was delaminated with cracks on the surface under extreme conditions due to rapid changes in the properties of the surface compared to the bulk. Based on the characterization of the aged rubber vulcanizate and the particles leached out into the aging fluid, a mechanism of HPHT aging of FKM vulcanizate is proposed, which would be helpful in designing fluorocarbon compositions in extreme environments.