Browning is an important factor affecting mushroom quality. To elucidate the molecular mechanism of nanocomposite packaging materials (Nano-PM) that delayed the browning of Agaricus bisporus, the metabolomic technology was applied. Thirty-five differential metabolites were obtained in A. bisporus packed with Nano-PM and commercial polyethylene packaging (control). Glutamate, leucine, and 3,4-dihydroxybenzaldehyde were detected as characteristic differential metabolites in Nano-PM treatment. According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis, amino acid synthesis and phenylalanine metabolism were enriched in mushrooms. In addition, glutamate, leucine, and 3,4-dihydroxybenzaldehyde inhibited tyrosinase (TYR) activity, leading to a better appearance and lower browning degree of packaged A. bisporus. Therefore, Nano-PM decreased mushroom browning by maintaining high levels of glutamate, leucine, and 3,4-dihydroxybenzaldehyde to inhibit TYR activity in A. bisporus.