There is a demand to develop techniques for the continuous removal/immobilization of heavy metals from contaminated soil and water bodies. In this study, a unique biochar preparation method was developed for the removal of cadmium. First, conventional biochars of corn straw were produced by pyrolysis at two temperatures and then treated using one-step synthesis at different ferric nitrate ratios and different calcination temperatures to produce magnetic biochars. Second, the prepared biochars were used as adsorbents for Cd(II) removal from a solution, and the best one was selected for further evaluation. Various techniques were used to characterize the adsorbents and determine the main adsorption mechanism. The results indicated that the biochars successfully carried iron particles within, which improved the specific surface area, formed inner-sphere complexes with oxygen-containing groups, and increased the number of oxygen-containing groups. The adsorption experiments revealed that MBC800-0.6 had a higher affinity for Cd(II) than the other adsorbents. Batch adsorption experiments were performed to explore the influence of the kinetics, isotherm, pH, thermodynamics, ionic strength, and humic acid on Cd(II) adsorption. The results indicated that the Langmuir model fit the Cd(II) adsorption best with MBC800-0.6 having the highest adsorption capacity (46.90 mg g). The sorption kinetics of Cd(II) on the adsorbent follows a pseudo-second-order kinetics model. Because MBC800-0.6 is loaded with metal ions, it can be conveniently collected by a magnet. Thus, biochar modification methods with ferric nitrate impregnation provide an excellent approach to eliminating Cd(II) from aqueous solutions. The possible adsorption mechanisms include chemisorption, electrostatic interaction, and monolayer adsorption.