A facile ball-milling method was developed to synthesize MgO/biochar nanocomposites as a dual-functional adsorbent. The physicochemical properties of the synthesized nanocomposites indicated that the composites achieved nano-scaled morphologies and mesoporous structure with MgO nanoparticles, which is approximate 20 nm and dispersed uniformly on the surface of the biochar matrix. Batch sorption experiments yielded 62.9% removal of phosphate, an anion, and 87.5% removal of methylene blue, a cationic organic dye, at low adsorbent dosages of 1.0 g L and 0.2 g L, respectively. This work indicates that ball milling, as a facile and promising method for synthesis of carbon-metal oxide nanocomposites, lends the advantage of operational flexibility and chemical adjustability for targeted remediation of diverse environmental pollutants.