In this study, nanocomposite thin films from cellulose nanocrystals (CNCs), zinc sulfide (ZnS) quantum dots, and polylactic acid (PLA) with fluorescent and antibacterial properties were constructed via a simple, and fast route. ZnS nanoparticles were added to CNC following the hydrothermal process. The film casting method was used to form thin nanocomposite films. The transmission electron microscope analysis revealed the uniform dispersion and ZnS nanoparticles through CNC suspension and FTIR results confirmed the interaction between –OH group on CNC and ZnS nanoparticles. A noticeable emission peak at 475 nm for CNC‐ZnS signifies the suitability of CNC‐ZnS for the assembly of ZnS QDs. The addition of ZnS‐coated CNC into PLA film resulted in a bright blue fluorescence when exposed to ultraviolet light. The incorporation of 5 wt% CNCs loaded with ZnS quantum dots into PLA notably decreased bacterial growth in Escherichia coli and Salmonella, respectively. Data obtained from dynamic mechanical analysis underlined that the presence of 1 wt% ZnS‐coated CNC in PLA increased the storage modulus of nanocomposite films by 25%. The CNCs loaded with ZnS quantum dots showed effective fluorescent and antimicrobial properties to be used as food packaging material.