High-performance carbon materials for energy storage applications have been obtained by using poly(m-phenylene isophthalamide), PMIA, as a precursor through the chemical activation of the carbonized aramid fiber by using KOH. The yield of the process of activation was remarkably high (25–40 wt %), resulting in activated carbon fibers (ACFs) with ultrahigh surface areas, over 3000 m2 g−1, and pore volumes exceeding 1.50 cm3 g−1, keeping intact the fibrous morphology. The porous structure and the surface chemical properties could easily be controlled through the conditions of activation. The PMIA-derived ACFs were tested in two types of energy storage applications. At −196 °C and 1 bar, H2 uptake values of approximately 3 wt % were obtained, which, in combination with the textural properties, rendered it a good candidate for H2 adsorption at high pressure and temperature. The performance of the ACFs as electrodes for electrochemical supercapacitors was also investigated. Specific capacitance values between 297 and 531 F g−1 at 50 mA g−1 were obtained in aqueous electrolyte (1 M H2SO4), showing different behaviors depending on the surface chemical properties.