Maghemite nanocrystals decorated multi-wall carbon nanotubes (MWCNTs) magnetic and electronic properties enhanced nanocomposites are synthesized using infrared (IR) irradiation and diazonium chemistry. The present method is simple and very effective as it overcomes the problem faced in the past decade for the decoration of carbon nanotubes (CNTs) with maghemite nanoparticles. The resulting materials are characterized using different techniques such as XPS, TEM, PXRD, STEM, EDX, HRTEM, and UV–Vis spectrophotometry. Firstly, sodium hydroxide treatment leads to the purification of MWCNTs (p-MWCNTs). The p-MWCNTs functionalization with tricarboxylic aryl diazonium salts generated in situ (p-MWCNTs-D3) followed by its impregnation with iron (II) ethylenediammonium sulfate (p-MWCNTs-D3/IEDS) utilizing IR radiation is the key step in homogeneously impregnating functionalized MWCNTs. Calcination of p-MWCNTs-D3/IEDS at 500 °C under argon atmosphere results in a controlled decoration of p-MWCNTs with maghemite nanocrystals (p-MWCNTs/MC). A homogeneous distribution of maghemite nanocrystals (cubic crystal system) on MWCNTs is observed in the size range of 1–6 nm, with a Gaussian mean diameter of ∼ 1.9 nm. To illustrate the applications of p-MWCNTs/MC for the decontamination of pollutants in water, methylene blue (MB), a model pollutant, is used and the performances are compared with the unmodified MWCNTs, Fe 2O 3 nanopowder and a mixture of MWCNTs and Fe 2O 3 nanopowder (1:1). Successful integration of properties of both constituents (MWCNTs and maghemite nanocrystals) in the new nanocomposite with superior characteristics is proved. The present method is valid for large-scale preparations which also opens very interesting perspectives in nanotechnology.