Constructing composites with heterogeneous structure and dual loss mechanism shows great potential in designing microwave absorbers. In this work, two-dimensional cobalt and nickel alloys@mesoporous carbon (CoNi@MC) composites were constructed via using CoNi layered double hydroxide@mesoporous polydopamine (CoNi LDH@MPDA) as sacrifice template. During the pyrolysis process, the MPDA is transformed into mesoporous carbon coated the surface of CoNi LDH that is further reduced to CoNi alloys. The mesoporous structure is conducive to the multi-reflection of electromagnetic waves and facilitates optimizing impedance matching. Heterogeneous interfaces between CoNi alloys and mesoporous carbon induce interface polarization. Multiple attenuation mechanism promotes the electromagnetic waves conversion. The maximum reflection loss of CoNi@MC composite is −70.86 dB and the widest effective absorption bandwidth is 7.74 GHz covering almost the entire Ku band. This strategy will be a guidance for designing electromagnetic absorbers.