Microwave absorbing materials (MAMs) have been identified as an efficient means to solve major electromagnetic pollution problems. Current core–shell composite MAMs are fabricated as single magnetic cores with dielectric shells, yielding decreased magnetic couplings and impedance mismatches. Herein, carbon shell encapsulated core–shell structured zinc ferrate (ZnFe₂O₄) sphere composites (CSZF@C) were fabricated using a hydrothermal method and subsequent carbonisation process. The complex permittivity and complex permeability of CSZF@C can be effectively adjusted by varying the parameters of the outer carbon shell. The synergistic effect of carbon shell and inner core–shell structured ZnFe₂O₄ (CSZF) not only meets impedance matching but also improves electromagnetic energy loss, a result of the unique microstructure. CSZF@C-1 exhibited a considerable reflection loss (RL) of −53.5 dB and an effective absorption bandwidth (EAB) of up to 6.56 GHz, the thickness is only 2.94 mm. Meanwhile, the epoxy resin coating of CSZF@C-1 substantially increases the corrosion resistance of the metal substrate owing to carbon encapsulation. This study presents new ideas for designing efficient multifunctional nanocomposites with high microwave absorption and corrosion resistance.