Agaricus bisporus (J. E. Lange) Imbach, well known as the common cultivated mushroom or white button mushroom, is widely cultivated all over the world. We used iTRAQ-MS/MS (i.e., isobaric tags for relative and absolute quantification-coupled two-dimensional liquid chromatography-tandem mass spectrometry) to identify protein expression changes that occur during the post-harvest maturation of Agaricus bisporus fruitbodies at 0, 6, 12, and 48 h after harvest. A total of 5878 unique peptides, representing 1063 proteins, were identified. Quantitative data were obtained from 1012 out of the 1063 proteins. A total of 102, 106, and 160 differentially expressed proteins (> twofolds differences) were identified from samples collected at 6, 12, and 48 h compared to sample from 0 h post harvest, accounting for 10.1, 10.5, and 15.8% of the total proteins identified, respectively. All identified proteins including the differentially expressed proteins among different post-harvest stages were subjected to bioinformatic analysis. Furthermore, seven representative proteins either up or down-regulated at different post-harvest stages were analyzed by real-time PCR. Three out of the seven proteins, the mismatched base pair and cruciform DNA recognition protein, hydrophobin-B and protein transporter, exhibited similar expression patterns as their corresponding proteins. The 260 differentially expressed proteins identified from our study laid a foundation for future studies aiming to understand the post-harvest maturation process of A. bisporus and eventually, might help in the development of breeding program to identify strains with extended shelf life.