Abstract: Composition optimization and micromorphology modulation have been shown to significantly enhance excellent electromagnetic wave (EMW) absorption. In this work, a series of V–Mn oxide/graphene composites with tunable phase compositions and distinct morphologies were synthesized by adjusting the VO2 to MnO2 ratio via a two-step hydrothermal method. By varying the MnO2 content, a directional phase evolution from VO2 to V2MnO6 and finally to Mn2V2O7 is achieved, accompanied by a morphological progression from nanocolumns to nanoflowers and ultimately to dense agglomerates. This phase and morphology engineering enables systematic control over the dielectric constant and impedance matching. Among the samples, the VM2 composite, featuring a V2MnO6-dominant phase and a self-assembled nanoflower architecture, exhibits optimal performance with an effective absorption bandwidth of 5.52 GHz and a minimum reflection loss of –58.4 dB. Radar cross section simulations further confirm its superior absorption capability. This enhancement in microwave absorption is attributed to a synergistic combination of moderate impedance matching, multiple internal reflections, interfacial and dipole polarization, and conduction loss. This work demonstrates that regulating phase composition is a promising route for tailoring the morphology and electromagnetic properties of dielectric microwave absorbers.