The thermophilic cellulase Cel5a from Fervidobacterium nodosum (FnCel5a) was conjugated with neutral, cationic, and anionic polymers of increasing molecular weights. The enzymatic activity toward an anionic soluble cellulose derivative, thermal stability, and functional chemical stability of these bioconjugates were investigated. The results suggest that increasing polymer chain length for polymers compatible with the substrate enhances the positive impact of polymer conjugation on enzymatic activity. Activity enhancements of nearly 100% were observed for bioconjugates with N,N-dimethyl acrylamide (DMAm) and N,N-dimethyl acrylamide-2-(N,N-dimethylamino)ethyl methacrylate (DMAm/DMAEMA) due to proposed polymer-substrate compatibility enabled by potential noncovalent interactions. Double conjugation of two functionally distinct polymers to wild-type and mutated FnCel5a using two conjugation methods was achieved. These doubly conjugated bioconjugates exhibited similar thermal stability to the unmodified wild-type enzyme, although enzymatic activity initially gained from conjugation was lost, suggesting that chain length may be a better tool for bioconjugate activity modulation than double conjugation.