There are 4 billion tons of uranium stored in seawater, providing energy equivalent to that from 7.963 × 10¹³ tons of coal combustion. However, due to low uranium concentration and competitive ions, uranium extraction from seawater has always been challenging. Metal composite nanomaterials with advantages like low cost and simple synthesis have been used as adsorbents, but the further application in uranium extraction was limited due to potential environmental impact, insufficient adsorption capacity, and selectivity. Therefore, in this study, a magnetic nanomaterial Mg-FeMeC (magnesium-functionalized ferro metal–carbon nanocomposite) was synthesized by a single-step solvothermal method, for uranium extraction. Experimental results showed that uranium adsorption percentage could reach 99.9% by using Mg-FeMeC-60% in solution with uranium concentration as low as 0.01 mmol/L. The adsorption selectivity was also confirmed by an adsorption efficiency as 79.0% and capacity as 118.54 mgU/g to uranium in solutions with copper ions interference. The U–Mg ion exchange was found as the most critical adsorption mechanism, together with favorable synergy effect of the sphere and sheet structures in Mg-FeMeC-60%. Moreover, strong interactions between uranium ions and the substrate may also help to alleviate the possibility of uranium release due to regular coordination adsorption, which can further avoid environmental risk of uranium leakage during practical application. Most significantly, the Mg-FeMeC-60% can almost completely adsorb the uranium in 25 L of collected seawater by only 0.1 g of the material. Therefore, with promising results for uranium extraction, the Mg-FeMeC-60% material is expected to be applied for marine uranium extraction to promote clean energy development.