Magnetic-functionalized multi-walled carbon nanotubes (MWCNTs) with encapsulated Fe nanoparticles have been successfully fabricated via a one-step decomposition approach using ferrocene as a single precursor at a high temperature of 950 °C. The constituents, phase, and morphology have been characterized by X-ray diffraction, Raman spectroscopy and transmission electron microscopy (TEM). Magnetic measurements reveal the ferromagnetic nature of the material with a magnetization saturation of 82.6 emu g⁻¹ at room temperature. Full nitrogen sorption isotherms show that the magnetic-functionalized MWCNTs have a porous structure (2.0 nm, 6.42 nm and 11.4 nm) and large surface area (295.4 m² g⁻¹). Owing to their wonderful intrinsic properties, these magnetic-functionalized MWCNTs exhibit excellent ability to remove heavy metal ions from aqueous solutions. For example, the removal efficiency can be up to 97% at a Cu²⁺ concentration of 100 mg L⁻¹ using these magnetic-functionalized MWCNTs as adsorbents. In addition, because the Fe nanoparticles were wrapped by MWCNTs, the material can be used in acidic solution and the removal efficiency of Cu²⁺ can reach 90% even in a strong acid solution (pH = 1).