Seven kinds of highly porous activated carbon were prepared from two different lignocellulosic biomass feedstocks (hybrid willow and miscanthus grass) by utilizing four different processing routes, which generally include variations of the pyrolysis, slow oxidative torrefaction, and KOH chemical activation. The activated carbons were evaluated for potential application within the electrodes of double layer supercapacitors. The synthesized activated carbons showed high specific surface area (up to 3265 m²/g), hierarchical pore structure composed of micro-/meso-/macropores with large pore volume (up to 1.535 cm³/g), and rich oxygen content (10.9–19.2 at. %). Their surface area, pore structure/volume, microstructure, and surface functional groups were highly influenced by processing routes, which in turn determined their electrochemical performance and stability. In particular, pretreating the biomass samples via slow oxidative torrefaction substantially increased their surface area, total pore volume, and meso-/micropore volume, and the surface chemistry of these materials showed a higher concentration of carboxyl groups. The performance of two-electrode symmetrical supercapacitors was evaluated in a 6 M KOH aqueous electrolyte. They exhibited relatively high specific capacitance of 70.2–162.3 F/g under constant current density of 100 mA/g, with a high cycling stability based on the capacitance retention of 95.1–99.9% after 1000 cycles. In addition, an increase of 25.0–62.2 F/g was achieved in specific capacitance by including the pyrolysis and/or slow oxidative torrefaction in the synthesis protocol. The sample (HW-D) that exhibited the best performance also maintained 94.1% of its specific capacitance after 5000 charge/discharge cycles at 100 mA/g. The synthesis strategies including the slow oxidative torrefaction pretreatment showed great promise for preparing low-cost, porous carbon materials from renewable biomass sources that are highly suitable for incorporation in supercapacitors and other electrochemical applications.