Spent lemongrass–derived nanocellulose (CNC) was embedded into a chitosan–glycerol film and tested as packaging for white button mushrooms (Agaricus bisporus). The reinforced film reduced weight loss, browning, and decay, helping mushrooms remain acceptable up to 6 days at 39 °F (vs. 3 days for plain chitosan film). The biofilm’s enhanced mechanical and barrier properties extend shelf life using waste-derived materials.