In the present study, the milled carbon fiber (MCF) and conductive polyaniline (PANI) are impregnated in the epoxy matrix to synthesize a lightweight nanocomposite, providing efficient microwave absorption in the X band. The PANI 3D hollow spheres are uniquely synthesized by removing the polystyrene (PS) core structure of PS/PANI composites. The MCF and as-synthesized PANI hollow spheres are characterized by Raman spectroscopy, X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). The FESEM analysis confirmed the formation of 3D hollow PANI conductive spheres, which exhibited numerous porosity and high roughness on PANI sphere surfaces. The XRD revealed that the size reduction of carbon fibers down to the micro level resulted into more graphitic carbon. The MCF- and PANI-impregnated epoxy nanocomposites are evaluated by reflectivity measurements for their microwave absorption capabilities, exclusively in the X band. The correlation of the results shows that the ratio of MCF and PANI fillers in the epoxy matrix can be customized to obtain the desired microwave absorption performance of the synthesized nanocomposite. A maximum absorption value of -49.3 dB (99.998% absorption) as well as 1.7 GHz -10 dB effective bandwidth is achieved using a nanocomposite sample of 1.8 mm thickness comprising 4 wt % MCF and 1 wt % PANI filler loadings. The strong microwave absorption in the present situation for a very thin sample using just 5% of the total filler results into quite a lightweight composite, which basically originates from the electric/dipolar polarization in the interfacial polarization dominant in the micron length fibers and hollow 3D PANI spherical structures. The hollow 3D morphology here improves the impedance matching of the resulting nanocomposite sample due to multiple reflections and scattering of EM waves inside the structure, which substantially increases the reflection loss of EM waves in the specified frequency band. The novelty of this work lies in synthesizing the PANI/MCF-based filler for the first time and using it to develop a lightweight nanocomposite, which is found to be quite an efficient microwave absorber, especially for X band applications.