Chromium-contaminated water is regarded as one of the biggest threats to human health. In this study, a novel magnetic mesoporous MnFeO@SiO-CTAB composite was prepared by a facile one-step modification method and applied to remove Cr(VI). X-ray diffraction, scanning electron microscopy, transmission electron microscopy, specific surface area, and vibrating sample magnetometer were used to characterize MnFeO@SiO-CTAB composites. The morphology analysis showed that the composites displayed a core-shell structure. The outer shell was mesoporous silica with CTAB and the core was MnFeO nanoparticles, which ensured the easy separation by an external magnetic field. The performance of MnFeO@SiO-CTAB composites in Cr(VI) removal was far better than that of bare MnFeO nanoparticles. There were two reasons for the effective removal of Cr(VI) by MnFeO@SiO-CTAB composites: (1) mesoporous silica shell with abundant CTA significantly enhanced the Cr(VI) adsorption capacity of the composites; (2) a portion of Cr(VI) was reduced to less toxic Cr(III) by MnFeO, followed by Cr(III) immobilized on MnFeO@SiO-CTAB composites, which had been demonstrated by X-ray photoelectron spectroscopy results. The adsorption of Cr(VI) onto MnFeO@SiO-CTAB followed the Freundlich isotherm model and pseudo-second-order model. Tests on the regeneration and reuse of the composites were performed. The removal efficiency of Cr(VI) still retained 92.4% in the sixth cycle. MnFeO@SiO-CTAB composites exhibited a great potential for the removal of Cr(VI) from water.