SiC-based composites are widely used as electromagnetic wave absorbers due to their excellent dielectric properties. However, the constraints associated with structural design and the intricacies of the preparation process hinder their broader application. In this study, novel mullite anti-gyroid/SiC gyroid metastructures are designed to integrate the mechanical and electromagnetic wave (EMW) absorption properties of composite materials. Mullite anti-gyroid/SiC gyroid composites are fabricated utilizing a combination of digital light processing (DLP) three-dimensional (3D) printing and precursor infiltration and pyrolysis (PIP) processes. Through the modulation of structural units, the electromagnetic parameters can be effectively regulated, thus improving the impedance matching characteristics of the composites. The structural composites show outstanding EMW absorption properties, with a minimum reflection loss of −54 dB at a thickness of 1.9 mm and an effective absorption bandwidth of 3.20 GHz at a thickness of 2.2 mm. Furthermore, the PIP process significantly enhances the mechanical properties of the composites; compared with those of the mullite/SiC ceramics, the flexural strength of the composites is improved by 3.69–5.85 times (13.28±1.15 MPa vs. (49.05±1.07)–(77.78±3.72) MPa), and the compressive strength is improved by 4.59–13.58 times (8.55±0.90 MPa vs. (39.02±1.63)–(116.13±2.58) MPa). This approach offers a novel and effective method for fabricating structural composites with an expanded range of higher electromagnetic wave absorption properties and improved mechanical properties.