Currently, as the electromagnetic (EM) environment becomes increasingly complex, single-function EM materials can hardly resist the increasing electromagnetic interference (EMI), and there is an urgent need to develop multifunctional EM materials. In this work, multifunctional WSe₂/Co₃C was prepared by simple hydrothermal methods. Its dielectric performance and EM response were investigated. Efficient absorption, shielding performance, and energy conversion devices were customized. By tailoring the loading content, WSe₂/Co₃C can switch between EM absorption and EMI shielding. The maximum shielding effectiveness (SE) of WSe₂/Co₃C reached 36 dB, and high reflection loss (RL) of −60.28 dB and wide effective absorption bandwidth (EAB) of 6.16 GHz can be obtained at low thickness. The multiple EM attenuation mechanisms brought by the combination of two-dimensional (2D) WSe₂ and magnetic Co₃C are considered to be the main reason for the enhanced EM attenuation ability. The WSe₂/Co₃C composite provides a viable candidate for developing multifunctional EM materials in 2–18 GHz.