Cadmium-containing wastewater draws worldwide attention due to its toxicity. In this work, a high-efficiency and eco-friendly MnO2-coated amorphous carbon (AG@MnO2) adsorbent for cadmium (Cd(II)) removal is synthesized using waste graphite anodes from spent lithium-ion batteries. Intensive mechanical energy is introduced to activate waste graphite through increasing the surface area by more than 25 times and the number of surface functional groups and defects. It qualifies AG@MnO2 for superior adsorption capability for heavy-metal ions after in situ loading of MnO2. High-energy ball milling preparation takes 6 h at the speed of 600 rpm, which increases the adsorption capacity from 4.88 to 135.81 mg/g. The removal of Cd(II) by AG@MnO2 is mainly by electrostatic attraction. This study provides a promising method to synthesize effective, low-cost, and eco-friendly adsorbents for Cd(II) removal by utilizing spent anodes. Furthermore, AG@MnO2 adsorbent can be further applied to remove other heavy-metal ions in wastewater.