Herein, Agaricus augustus, a mushroom, loaded on γ-Fe2O3 magnetic nanoparticles was developed as a biosorbent for the simultaneous preconcentrations of trace levels of Hg(II), Ni(II), and Zn(II)) in tap water, beverages, and some food samples prior to their determination by inductively coupled plasma optical emission spectrometry (ICP-OES). The experimental factors affecting the extraction performance were carefully examined throughout the investigation. The best experimental conditions were determined as pH 4.0, 3 mL/min of sample flow rate, 75 mg of biosorbent, 100 mg of γ-Fe2O3 magnetic nanoparticles, and 500 mL of sample volume. LOD values were determined as 0.016, 0.011, and 0.023 ng/mL, respectively for Hg(II), Ni(II), and Zn(II). The biosorption capacities were determined as 47.2 mg/g for Hg(II), 51.5 mg/g for Ni(II), and 49.0 mg/g for Zn(II). The effectiveness of the recommended method was assessed using certified reference material.