In general, the activation of lignin involves acid demineralization followed by a chemical treatment under an inert atmosphere at a high temperature. The whole process is tedious, time-consuming, and hazardous due to the suspended acid and related pH adjustment. Instead of the acid treatment, herein, we have employed a green technique, i.e., an ultrasonic treatment of the softwood Kraft lignin (SKL) followed by its activation under an inert atmosphere, and compared it with the acid-treated activated SKL. The particle sizes, microscopic structures, and surface properties of inactivated SKL, acid-treated activated SKL (AA-SKL), and ultrasonically treated activated SKL (UA-SKL) were investigated. The ultrasonic treatment reduces the processing time (especially the time required for pH adjustment) while producing activated carbon of comparable properties to that obtained using the traditional technique. Compared to the commercial carbon black (CB), UA-SKL efficiently adsorbs the noxious volatile organic compounds (VOCs), e.g., acrolein, xylene, etc., emitted from polyurethane (PU) composite foams to be used in automobile interiors. The present technique being simple and eco-friendly can be considered for industrial upscaling toward replacing the commercial CB by the biochar of lignin-like waste materials.