A novel magnetic biochar from sewage sludge (MSBC) using SrFe12O19 as magnetic substrate was successfully synthesized under high-temperature and oxygen-free conditions. Several techniques and methodologies (X-ray diffraction, Fourier transform infrared spectroscopy and vibrating sample magnetometer) were used to determine the surface functional groups and physicochemical properties of MSBC, which showed that the MSBC combined the features of both SrFe12O19 and sludge biochar (SBC). And then the adsorption behavior of methyl orange (MO) from aqueous solution onto the MSBC was investigated. And the influence of variables including pH, initial concentration of MO, adsorbent dosage and contact time was studied in detail. The optimal adsorption amount of MO (149.18 mg·g−1) was obtained with 600 MO mg·L−1, 2 MSBC g·L−1, at pH of 5 for 40 min. The equilibrium data were evaluated using Langmuir and Freundlich isotherms. The Langmuir model better described the absorption of MO. Besides, the kinetic data were analyzed using pseudo-first-order and pseudo-second-order equations, and the pseudo-second order exhibited the better fit for the kinetic studies (R2 = 0.9982). This study showed that MSBC could be utilized as an efficient, magnetically separable adsorbent for the environmental cleanup.