Given its easy-to-tailor microstructure with mixed nanodomains including amorphous and pseudo-graphitic parts, hard carbon (HC) as a type of non-graphitizable carbon promises to play a role in extended selection for novel metal/carbon microwave absorbers. Herein, we report the preparation of FeCoNi@HC composites as electromagnetic (EM) absorbers at different pyrolysis-controlled temperatures, derived from FeCoNi nanosheets (NSs)@polyimide (PI) composites. According to suitable pyrolysis treatments, FeCoNi@HC composites exhibit a wide range of mixed morphologies constituted of nanosheets and nanoparticles: original FeCoNi NSs are thermally converted into a particle-like structure as the core; the HC derived from the polyimide (PI) matrix inherits the original framework of the FeCoNi NSs for varying degrees of pyrolysis as the shell. The heat treatments also strongly influence the inner HC structural properties, including defect feature and graphitization degree. Treated by a moderate degree of pyrolysis, the optimal sample achieves decent enhancements of lossy attenuations (e.g., dielectric/magnetic losses) and promoted impedance matching for excellent EM absorption performances. The minimum reflection loss (RL) of the optimal sample is −61.12 dB, along with an effective absorption bandwidth (RL ≤ −10 dB) of 5.53 GHz. We anticipate that this work may enrich the survey of metals/carbon absorbers with non-graphitizable carbon supports.