Camellia oleifera fruit shell (COS) is an agricultural waste product generated in large quantities by the seed oil extraction industry. Due to its hierarchical thickness structure, COS shows huge potential in constructing porous carbon materials after thermal chemical modification. Herein, a series of COS biochars were synthesized by a carbonization-activation process and achieved excellent mercury removal performance in an aqueous environment. High-temperature carbonization was found to facilitate lignin removal and porosity generation, while retaining hydroxyl and carbonyl groups available for mercury adsorption. A volume of micropores of 594 × 10−3 cm−3/g with average pore diameter of 1.7 nm was achieved in activated COS biochar. At 550 °C, an adsorption capacity of 57.6 mg/g was realized in 1 mg/L Hg2+ solution under different pH environments. This work provides an alternative adsorbent for removing hazardous materials using sustainable bioresources.