Quaternary ammonium-functionalized magnetic mesoporous silica (AMMS) materials were prepared by the co-condensation method for the removal of Hg(II) from aqueous solutions in the presence of Cl⁻ ions. The quaternary ammonium moieties grafted on the AMMS frameworks strongly interacted with HgCl₃⁻ species to form ion pairs for the removal of Hg(II) from aqueous solutions. The formation of ion pairs was highly effective in aqueous solutions below pH 6, but above pH 6, hydroxide-related Hg(II) species became prevalent and substantially inhibited the formation of the ion pairs. In combination with magnetic separation, the AMMS adsorbents were regenerated with mild basic solutions at pH 8.5 for reuse in Hg(II) removal without performance reduction. The AMMS materials for Hg(II) removal through ion-pair formation have several advantages, including effective adsorption in the presence of Cl⁻, easy separation with a magnetic field, and complete regeneration under mild basic conditions.