This study explores the enhancement of mechanical and adhesive properties of polyester (PE) resins through the incorporation of bio-derived chitin nanowhiskers (CNWs) into the polymer matrix. CNWs are high-performance nanoparticles extracted from chitin, an abundant and renewable biopolymer. The research investigates the effects of processing strategies and CNW loadings on the chemical structure, thermal behavior, mechanical strength, and adhesive performance of PE–CNW nanocomposites. CNWs were incorporated into the PE matrix via slurry compounding using different suspension media (ethanol, acetone, and methyl ethyl ketone) and through direct mechanical mixing with CNW dry powders. Experimental results show that the thermal and mechanical properties of the nanocomposites are highly sensitive to both the thermal history during processing and the choice of suspension medium. Most importantly, the optimal adhesive performance was achieved via slurry compounding with a CNW suspension in ethanol, following an evaporative pre-treatment of the suspension to reduce ethanol content and thereby minimize transesterification of the polyester matrix.