The effective and rapid removal of hazardous dyes from wastewater remains a pressing environmental issue. Herein, we present the synthesis of hollow-spherical MgO/Mg(OH)2 nanomaterials via a simple, rapid, and surfactant-free microwave-assisted method using elemental magnesium as the precursor. Structural analyses confirmed the formation of uniform hollow spheres with a high specific surface area (176.3 m2 g−1) and mesoporous structure, which were strongly influenced by the H2O/H2O2 volume ratio and microwave power. The optimized material, synthesized with a high H2O2 content and elevated microwave power, exhibited exceptional catalytic performance, achieving the complete degradation of malachite green dye (200 μmol l−1) in a mere 1 min under ambient conditions. Furthermore, the material demonstrated excellent recyclability, maintaining high removal efficiency over five consecutive cycles. Mechanistic studies indicated that dye degradation is primarily attributed to the generation of hydroxide ions in an alkaline medium, facilitating a nucleophilic attack on the dye’s chromophore structure. The combination of rapid synthesis, high efficiency, and reusability makes the synthesized MgO/Mg(OH)2 nanomaterial a promising candidate for practical wastewater treatment applications.