Advancement in wireless technology has increased the usage of wireless devices extensively in the past few years, which led to an increase in electromagnetic interference (EMI) in the environment. Extensive research on fabrication of EMI shielding materials has been done. However, the role of processing method of polymer composites in EMI shielding has been neglected. In this work, we investigate the role of two polymer processing methods, spin coating and compression molding, in EMI shielding application. Poly(dimethylsiloxane) (PDMS) nanocomposites with multiwalled carbon nanotube (MWCNT) were spin-coated onto glass slides and compression-molded to a similar thickness. The processing method that exhibited the best shielding was employed to fabricate multiple PDMS composites comprising different compositions of MWCNT and FeO and stacked to form a multilayered EMI shielding PDMS composite. Scanning electron micrographs revealed that MWCNT in spin-coated composites are significantly more agglomerated than in the compression-molded film. Direct current conductivity and curing temperature were higher in compression-molded films as the filler formed a well-percolated network and hindered cross-linking of polymer chains. EMI shielding results revealed that spin-coated films demonstrated greater shielding effectiveness than compression-molded composites in the Ku-band (12-18 GHz). Individual agglomerates of MWCNT in spin-coated film attenuated incoming electromagnetic radiation more effectively than well-dispersed MWCNT in compression-molded films. Therefore, PDMS composites of different compositions of MWCNT and FeO nanoparticles were prepared through spin coating and stacked with a gradient of filler concentration, which resulted in maximum shielding of -28 dB, i.e., shielding more than 99% of incoming EM radiation by a 0.9 mm film.