3D hierarchically structured porous carbon composite short fibers were prepared by carbonizing in N-2 and activating in CO2 microcellulose fibers originally tethered with nano-TiO2. The TiO2 nanoparticles grew in situ on the cellulose surface and inside-out the body of cellulose during their original hydrolysis synthesis process assisted by microwave irradiation, creating sufficient adsorption sites of hydroxyl groups and exposing abundant interfaces after heat treatment. The obtained carbonized fibers displayed rapid adsorption of methylene blue, phenol, and Cr (VI) and exhibited high adsorption capacity, outperforming commercial activated carbon powders having 50% more surface area. Moreover, the composite effectively photocatalyzed the oxidation of phenol and the reduction of Cr (VI) under both UV-vis and visible light. The enhanced photocatalytic activity was believed to be due to (1) the doping of N atoms to the TiO2 crystal lattice resulting from the formation of Ti3+ and oxygen vacancies during the carbonization process and (2) the interfaces formed between TiO2 and the carbon, which promotes charge transfer and inhibit electron-hole recombination. The dual functional TiO2-carbon composite fibers made by this simple approach with many naturally endowed properties can also be repeatedly used after regeneration, opening up a sustainable opportunity for their versatile applications for both water and air decontamination.