A direct approach was presented to graft phosphonate groups on magnetic mesoporous carbon by an impregnation method with environmentally friendly precursors unlike the conventional methods involving a series of complicated steps and harsh conditions. Through the in situ reduction of Fe3+, magnetite particles of similar to 10 nm were successfully embedded into the mesopores, which was confirmed by HR-TEM. Surface characterization by X-ray photoelectron spectroscopy and Fourier transform infrared revealed phosphonate functional groups anchored through multidentate bonding with the surface of P-Fe-CMK-3. Due to the combined advantages of mesoporous pore size (5.5 nm), phosphonate ligands (1.42 mmol g(-1)), and magnetic sensitivity (5.20 emu g(-1)), this multifunctional adsorbent captured >85% of UO22+ within 5 min and the maximum adsorption capacity was 150 mg g(-1) at pH 4. The exceptionally high selectivity and efficiency of P-Fe-CMK-3 toward uranyl capture even in groundwater (K-d = 1 x 10(5) mL g(-1)), radioactive wastewater (K-d = 3 x 10(4) mL g(-1)), and seawater (K-d = 1 x 10(4) mL g(-1)) at V/m = 1000 mL g(-1) was better than that of the previously reported adsorbents. Importantly, the adsorbent maintained UO22+ adsorption efficiency >99% over five cycles due to the excellent chemical and structural stabilities. Above all, the adsorbent could be manipulated for UO22+ capture with help of a magnetic field in the real world, especially in case of nuclear accidents, decommissioning of nuclear power plants and/or uranium recovery from seawater.