Hg(II), as a hazardous water pollutant, is attracting increasing attention due to its severe ecological and biological toxicity. Herein, a magnetic composite MXene/Fe3O4/FAC for aqueous Hg(II) uptake was prepared. It appeared fast adsorption kinetics (∼30 min of adsorption equilibrium time), high Hg capacity (924.34 mg/g), broad working pH range (pH=4–12), and high selectivity. Besides, the implanted Fe3O4 empowered the composite with good recycling and reusing characteristics, effectively preventing the release of nanomaterials into the environment. Multiple adsorption models and characterizations revealed the Hg(II) adsorption behaviors and mechanisms and the impact of Fe3O4 on Hg(II) adsorption onto MXene. Specifically, Hg(II) reductive adsorption aroused by the low-valence Ti species (abbreviated as Ti*) is the foremost mode of MXene. With the increasing of Fe3O4 (especially trivalent Fe), Ti* tends to be oxidized and covered by Fe3O4, and less Ti* can be used for Hg(II) reductive adsorption, leading to slower adsorption kinetics and lower adsorption capacity. The introduced inert carrier FAC favors the good dispersion of Fe3O4 and MXene to alleviate the oxidation of MXene, achieving the high utilization of MXene materials. The new insight into the interaction between MXene and Fe3O4 can beneficially guide the synthesis of Hg(II) magnetic adsorbents and MXene-based functional materials.