<p>The intensive development of radio-electronic technology has resulted in additional electromagnetic (EM) pollution and problems with EM interference, which has led to the creation of EM absorbing materials. This research presents the development of new polymer composites based on a filler that combines the advantages of the electrophysical properties of multi-walled carbon nanotubes (CNTs) with the magnetic tunability of substituted spinel ferrites (NiZn)_1−<i>x</i>Mn_<i>x</i>Fe₂O₄ (<i>x</i> = 0, 0.25, 0.5, 0.75 and 1) and a crystalline polychlorotrifluoroethylene matrix, designed for effective absorption of EM microwave radiation. The (NiZn)_1−<i>x</i>Mn_<i>x</i>Fe₂O₄ /CNT fillers (0 ≤ <i>x</i> ≤ 1) were synthesised by co-precipitation, and their spinel crystal structure was confirmed by X-ray diffraction and Raman spectroscopy. The magnetic properties of (NiZn)_1−<i>x</i>Mn_<i>x</i>Fe₂O₄/CNT fillers demonstrated typical superparamagnetic characteristics for single-domain nanoparticles, with maximum saturation magnetization values of 74.1&#xa0;emu/g at <i>x</i> = 0.75. The absorption, reflection and transmission coefficients (1–40&#xa0;GHz) of the fillers and polymer composites based on them, as well as the reflection coefficient on a metal plate, were calculated by the measured complex dielectric permittivity and complex magnetic permeability in the high-frequency range using the waveguide method. It has been shown that the introduction of a filler into a polymer matrix in an amount of 0.0025 to 0.015 volume fractions allows control their absorption properties, as well as shifting the frequency range of the maximum peak values of the absorption coefficient. The highest absorption coefficients, both in free space and on a metal plate, were observed for three-component systems at <i>x</i> = 0.5. </p> Graphical abstract <p></p>