Carbon cloth (CC) is an inexpensive and highly conductive textile with excellent mechanical flexibility and strength; it holds great promise as an electrode material for flexible supercapacitors. However, pristine CC has such a low surface area and poor electrochemical activity that the energy storage capability is usually very poor. Herein, we report a green method, two-step electrochemical activation in an aqueous solution of inorganic salts, to significantly enhance the capacitance of CC for supercapacitor application. Micro-cracks, exfoliated carbon fiber shells, and oxygen-containing functional groups (OFGs) were introduced onto the surface of the carbon filament. This resulted in an enhancement of over two orders of magnitude in capacitance compared to that of the bare CC electrode, reaching up to a maximum areal capacitance of 505.5 mF cm⁻² at the current density of 6 mA cm⁻² in aqueous H₂SO₄ electrolyte. Electrochemical reduction of CC electrodes led to the removal of most electrochemically unstable surface OFGs, resulting in superior charging/discharging rate capability and excellent cycling stability. Although the activated CC electrode contained a high-level of surface oxygen functional groups (∼15 at%), it still exhibited a remarkable charging–discharging rate capability, retaining ∼88% of the capacitance when the charging rate increased from 6 to 48 mA cm⁻². Moreover, the activated CC electrode exhibited excellent cycling stability with ∼97% capacitance remaining after 10 000 cycles at a current density of 24 mA cm⁻². A symmetrical supercapacitor based on the activated CC exhibited an ideal capacitive behavior and fast charge–discharge properties. Such a simple, environment-friendly, and cost-effective strategy to activate CC shows great potential in the fabrication of high-performance flexible supercapacitors.