Cellulose, the most abundant polymer on Earth, and its composites have recently gained importance for the production of sustainable materials. These materials should be produced using green methods that avoid the utilization of toxic chemicals to ensure integrity for environmental sustainability. Ball milling, which gives a straightforward and (often) green synthetic access to materials, can be used to achieve this goal. Previously, it was shown that mechanochemical bond breakages in polymers generate mechanoradicals, which can be used to drive further reactions and to form polymer composites. In this study, we show that cellulose mechanoradicals generated during the ball milling of cellulose can reduce various metal ions to the corresponding metal nanoparticles (NPs) (Au, Ag, Pt, Pd, Co, and Cu), which are deposited and stabilized in the cellulose matrix. Using mechanoradicals to reduce the metal ions and form the cellulose composites, (1) the number of synthetic steps is reduced, and (2) the conventionally used, toxic reducing and stabilizing agents are avoided, which also prevents the contamination of the composites. The cellulose-metal nanoparticle composites can exhibit a wide range of properties that depend on the metal nanoparticle in the composite; e.g., Au-cellulose nanocomposites exhibit catalytic activity, and Ag-cellulose nanocomposites exhibit antibacterial properties. The ball-milling method also permits blend formation using synthetic polymers, which allows tuning the physical properties of the final material. Finally, the method shown here provides a quick access to versatile metal nanoparticle cellulose composites (and their blends), which may find applications, such as in paper-based diagnostics and catalysis.