A magnetite-loaded mesocellular carbonaceous material, Fe3O4/MSU-F-C, exhibited superior activity as both a Fenton catalyst and an adsorbent for removal of phenol and arsenic, and strong magnetic property rendering it separable by simply applying magnetic field. In the presence of hydrogen peroxide, the catalytic process by Fe3O4/MSU-F-C completely oxidized phenol and As(III) under the conditions where commercial iron oxides showed negligible effects. Notably, the decomposition of H2O2 by Fe3O4/MSU-F-C was not faster than those by commercial iron oxides, indicating that hydroxyl radical produced via the catalytic process by Fe3O4/MSU-F-C was used more efficiently for the oxidation of target contaminants compared to the other iron oxides. The homogeneous Fenton reaction by the dissolved iron species eluted from Fe3O4/MSU-F-C was insignificant. At relatively high doses of Fe3O4/MSU-F-C, total concentration of arsenic decreased to a significant extent due to the adsorption of arsenic on the catalyst surface. The removal of arsenic by adsorption was found to proceed via preoxidation of As(III) into As(V) and the subsequent adsorption of As(V) onto the catalyst.