Graphene nanosheets (GNSs) were prepared by an efficient liquid-phase exfoliation method, and then the NiCo/GNS nanohybrids were fabricated using the single-mode microwave-assisted hydrothermal technique. The NiCo/GNS composites with different GNS proportions were investigated as microwave absorbers. Morphology investigation suggested that NiCo nanocrystals were uniformly anchored on the GNS without aggregation. The electromagnetic parameters of NiCo/GNS nanohybrids could be artificially adjusted by changing the GNS proportion, which led to an exceptional microwave-absorbing performance. A reflection loss (RL) exceeding -20 dB was obtained in the frequency range of 5.3-16.4 GHz for the absorber thicknesses of 1.2-3.2 mm, while an optimal RL of -30 dB was achieved at 11.7 GHz for a thickness of 1.6 mm. The enhanced microwave-absorbing performance indicated that the NiCo/10 wt % GNS composite has great potential for use as an excellent microwave absorber.