Spherical nickel–carbon (Ni–C) monoliths have been successfully synthesized via a solvothermal route and subsequent heat treatment without any templates, which offers a green and easier method to synthesize carbon-based materials. The macroscopical size of the spherical Ni–C monoliths is about 1 cm in diameter and the Ni nanoparticles are uniformly dispersed on the carbon matrix on a microscopic level. The resultant Ni–C monolith exhibited strong microwave absorption in the microwave range of 2–18 GHz at various loadings of nickel (10–30 wt%). Among these, the composite with an 18.2 wt% loading of Ni has a strong absorption peak at 13.7 GHz and achieves a maximum absorption value of 23.6 dB. More importantly, good reflection loss over a wide frequency range can be simply achieved by manipulating the absorber thickness, suggesting that these composites may be used as light-weight and highly effective microwave absorbers.