We report a simple, consistent, and economical wet-chemical method for synthesizing a series of Fe-Ni nanocomposites by varying Fe and Ni fractions. Further, graphite wrapped Fe-Ni alloy nanoparticles in core-shell geometries are prepared by calcination of nanocomposites at 900°C under N2 gas. XRD studies reveal body-centered and face-centered (bcc/fcc) cubic Fe-Ni alloy core particles and hexagonal graphitic shell formation. More interestingly, a structural transition from bcc to fcc phase is also investigated with varying Ni concentration. The microstructural studies demonstrate ~10 nm graphitic shells together with a nearly spherical ~40 nm diameter Fe-Ni metallic core. These magnetic alloys showed the variation in magnetization and coercive fields with alloy compositions. The Fe-Ni core-shell material with the maximum magnetization is used for fabricating rubber-based composites by varying filler loading in the range 60–80 wt%. The utility of Fe-Ni/C alloy and rubber composite is categorized in terms of electromagnetic impedance matching for the entire X-band (8.2–12.4 GHz) frequency range based on the filler concentration and absorber thickness for microwave absorption applications. These core-shell alloy systems may show potential for stealth of strategic targets because of their very low density (~1.1 g/cc) and lesser thickness than conventional absorbers. © 2021