Few studies have focused on the effects of biomass feedstocks on the structure-reactivity of magnetic biochar. In this study, four types of magnetic biochar were first prepared using steel pickling waste liquor (serving as iron salt) and sugarcane bagasse (SMBC), rice straw (RMBC), peanut shells (PMBC) and herb residue (HMBC). According to the Langmuir model, the maximum adsorption capacity of Cr(VI) by SMBC was 43.122 mg/g, which was approximately 1.298 times, 3.175 times and 3.677 times higher than that of RMBC, PMBC and HMBC, respectively. The mass balance of chromium indicated that Cr(VI) was removed mainly via reduction. However, the reduction capacity of Cr(VI) differed among the four types of magnetic biochar. The results of characterisations of those magnetic composites before and after reaction demonstrated that the Cr(VI) was electrostatically attracted to the surface of the materials. Consequently, most of the adsorbed Cr(VI) was reduced and the remainder of the Cr(VI) was complexed with C=O groups in magnetic biochar. Further, the total iron in magnetic biochar, especially Fe(II), played a dominant role in the removal and reduction of Cr(VI). Finally, correlation analysis showed that the cellulose and ash in the biomass were the main factors that induced the differences in the magnetic biochar's total iron content. Therefore, this study may provide a reference for the use of magnetic biochar to remove Cr(VI) from aqueous solutions.