Arsenic is a highly toxic metal that causes cancer even at a low concentration and its removal from water resources is challenging. Herein, carbon extracted from waste onion bulbs is activated to cater for porosity and functionalized with magnetite (Fe₃O₄) nanoparticles (named MCK6) to address the challenge of As(III) removal. Synthesized MCK6 was highly mesoporous having a surface area of 208 m²/g, where magnetite nanoparticles (≤ 10 nm) are homogeneously distributed within a porous network. The developed adsorbent inherited functional groups from the biosource and magnetic property from magnetite making it ideal for removal of As(III). Further, MCK6 showed a maximum monolayer adsorption capacity (q_m) of 10.2 mg/g at 298 K and pH 7. The adsorption thermodynamics delineates a non-spontaneous and endothermic reaction, where the kinetics followed pseudo 2nd order (R² value of 0.977), while monolayer formation is explained by the Langmuir model. Moreover, MCK6 efficiently works to remove As(III) in a competitive metal ions system including Pb⁺², Cd⁺², and Ca⁺², making it a suitable adsorbent to tackle contaminated water.