In this work, spinel nanoMnNiZn ferrite (Mn 0.1 Ni 0.45 Zn 0.45 Fe 2 O 4) was hosted by a thermoset polyurethane matrix (PU), to produce flexible thin nanocomposite sheets as efficient microwave absorbers. The microwave absorbers of polyurethane nanocomposites were prepared using different loading ratios of MnNiZn ferrite (70%, 75%, and 80wt%). Microwave absorption properties of the nanocomposites were characterized in the range of Sand C-band (2-8 GHz) frequency. Structural and morphological characterizations were performed using X-ray diffractometry (XRD), Fourier-transform infrared spectrophotometer (FTIR), and scanning electron microscopy (SEM). The tensile test was performed to examine the mechanical properties of the molded nanocomposites. The surface hardness was measured using Shore (A) hardness tool. The nanocomposite with 80 wt% loading percent of ferrite in PU at thickness of 5 mm exhibits the best absorption properties and has effective absorption bandwidth (≥ − 10 dB) of 4.28 GHz, with − 29.7 dB minimum reflection loss (RL) at the matching frequency (5.86 GHz) with density of only 2.403 g/cm 3. The experimental results reveal that the microwave absorption properties of the synthesized nanocomposites were improved and reinforced in the case of polyurethane matrix compared to the paraffin wax matrix, through polar-polar interactions and adhesion between the filler and elastomeric matrix depending on their chemical nature. Also, the effect of thermal aging on the performance of the microwave absorption properties of PU-ferrite nanocomposite has been studied. To the best of our knowledge, the use of this cost-effective thermoset polyurethane matrix prepared from PPG, TDI, and castor oil and loaded with ferrite has not been reported before. The effect of polar-polar interactions between the matrix and the ferrite on the microwave absorption characteristics has also been investigated. The high value of reflection loss makes the obtained lightweight and flexible nanocomposite a promising microwave-absorbing material.