Rational design on the microstructure of electromagnetic composites offers immense potential for overcoming the challenges related to the microwave absorption performance. In this study, uniform core-shell Co@C microspheres are innovatively fabricated through an in situ transformation from Co3O4@phenolic resin precursor. Carbon shells restrain the agglomeration of Co particles during high-temperature treatment, which accounts for the survival of uniform core-shell microstructure. Electromagnetic analysis reveals that Co@C microspheres show quite different electromagnetic functions as compared with pure Co derived from Co3O4 microspheres. On one hand, carbon shells can effectively regulate the complex permittivity of Co@C microspheres by reducing conductivity loss and introducing polarization relaxations; on the other hand, carbon shells suppress the skin effect from cross-linked Co networks and make the isolated Co cores produce stronger magnetic loss. The microwave absorption properties are evaluated in the frequency range of 2.0–18.0 GHz, and as expected, core-shell Co@C microspheres exhibits excellent reflection loss characteristics, where strong reflection loss (−68.7 dB at 10.6 GHz), ultra-wide response bandwidth (2.7–18.0 GHz over −10 dB), and thin matched thickness (1.65 mm) can be achieved. A delta-function method and attenuation constant validate that the matched characteristic impedance and improved loss ability in Co@C microspheres account for the significant enhancement. Display omitted