Poly(p-phenylene benzobisoxazole) (PBO) fibers are some of the strongest organic polymer fibers, however, even stronger fibers can be made using carbon nanotubes (CNTs). In the present study, the copolymerization of functionalized CNTs with PBO was successfully carried out by one-pot in situ polycondensation in polyphosphoric acid. Pristine CNTs were first oxidized to CNT–COOH and then modified by 4,6-diaminoresorcinol (DAR) or PBO trimer (DAR–TA–DAR) to improve the reactivity, solubility, dispersivity, and interfacial adhesion of CNTs in polymer matrix. Functionalized CNTs were further covalently incorporated with PBO through in situ polymerization. The chemical structure and morphology of functionalized CNTs and CNT–PBO copolymers were well characterized for confirming the formation of covalent bond between CNTs and PBO. Moreover, continuous CNT–PBO copolymer fibers have been fabricated using a dry-jet wet-spinning technique. The structure and morphology of CNT–PBO copolymer fibers have been characterized and their mechanical and thermal properties have been investigated. The tensile modulus, tensile strength, and thermal stability of CNT–PBO copolymer fibers have been improved. The improvement in PBO molecular chain packing order, the good dispersion and high alignment of CNTs in the PBO matrix, and the covalent bonding at the CNT–PBO interface have been suggested as the main reasons for the performance improvement of PBO.