The textile industry is currently facing immense pressure to transition towards sustainable development. However, waste textiles and fibers are still mainly disposed of in landfills or by incineration after their service life and thereby pollute the natural environment. In the study, we extracted keratin from various waste textiles and used it as a raw material for sustainable fiber production. We investigated how the amino acid composition of keratin is related to its hydrophilicity and molecular weight. Additionally, we compared the dissolution of regenerated keratin and waste textiles in ionic liquid. The viscoelastic properties of the keratin solution were characterized using small angel oscillation shear rheology. The results showed that the keratin solution is not suitable for dry-jet wet spinning. Furthermore, the extruded fibers were too weak to stretch in the fiber line. We blended high molecular weight cellulose with keratin during the dissolution process to regulate its viscoelasticity and the mechanical properties of the regenerated fibers. Interestingly, due to the absence of the complex structure found in textile waste and its inherent chemical stability, regenerated keratin demonstrated good compatibility with cellulose. These hybrids possess excellent mechanical properties (41.5 cN/tex) and a smooth, homogeneous structure. In this system, the reconfiguration of the keratin secondary structure and crystallinity of cellulose were investigated as the relationship between mechanical strength and chemical structure. This work offers insight into solving current challenges for sustainability about textile waste recycle and continuous manufacture of regenerated cellulose/protein fiber.