Numerous strategies involving multiple cross-linking networks have been applied for fabricating robust hydrogels. Inspired by this, the development of mechanically strong and tough biological fibers by the incorporation of intermolecular linking networks is becoming important. Herein, we present a versatile strategy for the fabrication of protein-saccharide composite fibers through protein-initiated double interacting networks. Three types of lysine-rich bioengineered proteins were introduced and the present multiple cross-linking interactions including electrostatic forces and covalent bonds significantly enhanced the mechanical properties of as-obtained composite fibers. In stark contrast to pristine saccharide or other polymer fibers, the as-obtained composite fibers exhibited outstanding mechanical performance, showing a breaking strength of ~768 MPa, Young’s modulus of ~24 GPa, and toughness of ~69 MJ∙m–3, respectively. Thus, this established approach has great potentials to fabricate new generation renewable biological fibers with high performance.