To address the challenge of textile waste upcycling, this study demonstrates a sustainable, acid-assisted hydrothermal strategy to transform waste cotton fibers into high-value functional carbon materials. By tuning reaction kinetics, we achieved the controlled synthesis of uniform submicron carbon spheres at 180°C for 4 h and novel silver/carbon (Ag/C) submicron wires by extending the process to 7 h. Morphological and structural analyses, including SEM, TEM, and SAED, confirm that the Ag/C composites possess a unique core/sheath architecture featuring quasi-single-crystalline silver cores encapsulated within a graphitized carbon matrix. Surface analysis via XPS and FT-IR reveals that strong interfacial interactions and a high density of oxygen- and sulfur-containing functional groups contribute to the material’s enhanced surface reactivity. Performance benchmarks in methylene blue remediation show that the Ag/C submicron wires significantly outperform pristine carbon spheres in both adsorption kinetics and removal capacity, driven by the synergistic effect between the silver core and the functionalized carbon shell. These results highlight a robust and scalable pathway for engineering high-performance adsorbents from low-value textile waste for advanced environmental remediation.