Metal-C nanoparticles with MnFe nanocores and their microwave absorption performances with design Mn/Fe ratios have been rarely reported in previous studies. In this work, MnFe-C core–shell nanoparticles with tunable Mn/Fe ratios were prepared by a one-step metal organic chemical vapor deposition method. It was found that the magnetic properties and microwave absorption performances of the nanoparticles could be well tunable by designing Mn/Fe ratios in their nanocores. These results suggest that the as-synthesized MnFe-C core–shell nanoparticles have great potential for microwave absorption applications.