A series of functionalized mesoporous carbon (MC) materials were prepared by covalent grafting with amidoxime, carboxyl, and phosphoryl functional groups and screened for uranium (U) sorption from aqueous media. The MC materials were characterized by nitrogen adsorption measurements and thermogravimetric analysis and investigated for U(VI) extraction from both acidic water (pH 4) and artificial seawater (pH 8.2). Under both conditions, a phosphoric acid-functionalized MC exhibited the highest U(VI) sorption, with a maximum sorption capacity of 97 mg of U(VI)/(g of sorbent) in acidic water and 67 mg of U(VI)/(g of sorbent) in artificial seawater. The effects of pH on U(VI) sorption, sorption kinetics, and sorption isotherms were obtained for the phosphoric acid-functionalized MC. Quantitative U(VI) removal from U(VI)-loaded sorbents was achieved by washing with HCl at concentrations higher than 0.01 M. These results indicate that functionalized MC provides a promising platform for the development of novel sorbents for efficient U(VI) extraction.