•A hierarchically porous carbon functionalized with Fe-Mn oxide was fabricated for superior removal of Hg(II) from water.•The composite exhibits high adsorption efficiency of 96.8% and fast adsorption rate for the removal of Hg(II).•The high efficiency is contributed by the synergy between physical and chemical adsorption.•The adsorption kinetics agrees with the pseudo-second order model, and isotherms are consistent with the Freundlich model. The removal of heavy metal ions from industrial wastewater by adsorption has been central to the environment for decades, where common adsorbent materials are often limited by poor efficiency, complex fabrication and long processing time. Porous carbon derived from biospecies holds promise to address the limitations. In this study we converted bagasse into a carbon composite having hierarchically porous structure; the composite’s dispersion phases – iron oxide and manganese oxide – were synthesized by a simple one-step liquid-phase reaction method. Featuring large specific surface area of 350.8 m2 g−1, the composite demonstrated exceptional Hg (II) removal efficiency of 96.8%, adsorption rate of up to 96.8% within 150 min and adsorption capacity of 9.8 mg g−1. In comparison with other removal materials, our work is outstanding in terms of both removal efficiency and synthesis simplicity. The high efficiency is attributed to the synergy between physical adsorption referring to hierarchically porous structure and chemical adsorption relating to functional complexation processes. It provides a new avenue for the development of high-performance adsorbent materials for heavy metal removal from aqueous media.