Lithium/sodium (Li/Na) metal batteries (LMBs/SMBs) have emerged as frontrunners for next-generation energy storage systems due to their ultrahigh theoretical energy densities and the natural abundance of Li and Na. However, their practical deployment is impeded by critical challenges, including dendrite growth, dead Li/Na formation, and severe volume expansion, which markedly degrade battery performance and shorten cycle life. Recent advancements have focused on 3 strategic approaches: composite anode design, electrolyte formulation, and artificial solid-electrolyte interphase (SEI) engineering. Among these, carbon nanostructured materials have garnered particular attention due to their large specific surface area, excellent electrical conductivity, tunable pore architecture, and mechanical robustness. This review systematically dissects the failure mechanisms of LMBs/SMBs and presents a taxonomy of carbon-integrated solutions across molecular-to-macroscopic scales. Particular focus is given to carbon materials such as graphene, carbon nanotubes, and carbon nanofibers, highlighting their roles as hosts, interlayers, and SEI regulators in suppressing dendrite formation and stabilizing electrode–electrolyte interfaces. Furthermore, the structural and chemical engineering of nanocarbon frameworks is discussed in terms of their contributions to enhanced cycling stability, improved Coulombic efficiency, and extended battery lifespan. This review concludes with a forward-looking perspective that prioritizes atomic-level interface tailoring, bio-inspired multidimensional architectures, and sustainable large-scale synthesis, aiming to accelerate the commercial deployment of carbon nanomaterials in LMBs/SMBs.