Using, as starting materials, fluid catalytic cracking decant oil (FDO), rich in short-chain alkyl groups, and synthetic naphthalene pitch (NP), with stable naphthenic structures, the synthesis of a spinnable mesophase pitch via a co-carbonization process was investigated. The effects of NP addition and the precursor molecular structure on properties of the resultant mesophase pitches and their carbon fiber derivatives were also discussed. With the increase of NP inclusion from 10 to 30 wt %, the solubility in toluene and quinoline and the optically anisotropic domain size of the synthesized pitches increase. However, the softening points of the resultant mesophase pitches decrease. In comparison to FDO, more naphthenic structures are retained in NP, and these show higher thermal stability during the preparation of mesophase pitch. The interaction of naphthenic structures in NP and short-chain alkyl groups in FDO promotes an increase in the molecular weight of the mesophase pitch prepared via co-carbonization and, in the present case, also increases the orientation and domain size in the resulting mesophase liquid crystal. The synthesized mesophase possesses a low softening point, good solubility, and 100 vol % mesophase content. Carbon fibers prepared from the co-carbonized mesophase pitch exhibit higher thermal conductivity than that of K-1100.