In recent years, more and more attention has been paid to the hydrothermal liquefaction (HTL) of waste biomass for the production of bio-oil and hydrochar (a solid residue from HTL process). However, hydrochar possesses limited porosity and surface area, hindering its environmental application. In the present work, to promote the development of a sustainable application of waste biomass, waste hydrochar was activated and modified to a novel magnetic carbon composite, which exhibited high performance for dye removal from aqueous solutions. The composite possessed a saturation magnetization of 38.5 emu g–1 at room temperature and could be facilely attracted from an aqueous solution by an external magnet. The as-prepared composite exhibited a superior malachite green (MG) adsorption capacity (476 mg g–1), which was much higher than the known magnetic adsorbents. Our results suggested that the waste hydrochar could be efficiently transformed to a high-performance sustainable material for dye removal.