To provide a theoretical basis for the development of future mercury removal methods, two types of biochars modified by the coprecipitation method were investigated: undoped iron-based biochars modified with FeCl3 and iron-based biochars doped with Cu and Mn from CuSO4, Mn(CH3COO)(2), and KMnO4. The crystal phase compositions, pyrolysis characteristics, pore structures, microscopic morphologies, elemental speciations, and functional groups of the modified biochars were characterized. The adsorption mechanism was further explored by temperature-programmed desorption (TPD). The results showed that the mercury adsorption of the modified biochars was significantly enhanced. The mercury adsorption performances first increased and then weakened with increased loading. The modification decreased the graphitization degree. Spinel structure solid solutions of MnFe2O4 and CuFe2O4 were formed in the modified biochars, generating many cation vacancies on the biochar surface. After biochar modification, the pore structures and the contents of carbonyl, carboxyl, and metal hydroxyl functional groups were significantly increased. The removal of Hg-0 by modified biochar is the result of a combination of adsorption and oxidation. The modification process significantly promotes the chemical adsorption of Hg-0. Functional groups, lattice oxygen, chemisorbed oxygen, halogen components, and metal oxides or ions on the sorbent surface all play roles in the oxidation of the adsorbed HG(0). Fe2O3 and CuO (MnO2 and Fe2O3) double metal oxides have a synergistic effect on the removal of Hg-0.