Abstract: The advancement of wireless technologies has increased the global demand for ubiquitous connectivity. However, this surge has increased electromagnetic pollution. This study introduces a composite comprising a polymer matrix (polydimethylsiloxane, PDMS) and a magnetic filler (carbonyl iron powder, CIP) to effectively absorb electromagnetic waves (EMW) and suppress electromagnetic noise, while exhibiting good mechanical properties. Eutectic gallium–indium (EGaIn) liquid metal (LM) was introduced to improve the insulating properties of magnetic fillers. A core–shell structure was obtained by coating the CIP particles with EGaIn, thereby combining magnetic and dielectric materials to enhance EMW absorption. The fluid characteristics of the LM improved the mechanical properties, whereas its electrical conductivity enhanced interfacial polarization loss, thereby augmenting the dielectric loss value of the composites. Moreover, the application of mechanical strain enhanced the EMW absorption of the LM/CIP/PDMS composites due to the formation of a conductive LM network.