Two novel magnetic biochar composites (FeC-H and FeC-P) were synthesized using corn stalks and ferrous sulfate through hydrothermal method and traditional pyrolysis, respectively. The samples were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), vibrating sample magnetometer, and particle size analyzer. Batch experiments were conducted to investigate the ability of those samples to absorb aqueous phosphate. FeC-H had lower surface area than FeC-P, but more hydrophilic functional groups were detected on the rough surface of FeC-H. The impregnated iron was present as Fe3O4 in the prepared magnetic biochar composites. The introduced Fe3O4 resulted in high performance of magnetic separation and also played the role as adsorption sites for phosphate. FeC-H and FeC-P demonstrated higher sorption capacity than bare Fe3O4 due to the highly dispersed and smaller crystalline sizes of Fe3O4 particles loaded in corn stalk derived-biochar support. FeC-H had the best performance, with Langmuir adsorption capacity as high as 5.04 mg/g for phosphate. These results indicate that the magnetic biochar composites prepared from corn stalks by hydrothermal method (FeC-H) have potential as a high-efficiency and cost-effective adsorbent for phosphorus removal from wastewater.