The globally cultivated white button mushroom (Agaricus bisporus) is grown commercially at an industrial scale. Numerous pathogens pose a significant economic threat to its cultivation. Due to the emergence of resistance towards fungicide treatments, the future of mushroom disease treatment will need to move towards integrated pest management including the use of biological control agents (BCAs). In this study, we investigated the impact of the BCAs, B. velezensis QST 713 and Kos on the population dynamics of the microbiota of mushroom casing. Amplicon sequencing revealed that four Bacterial phyla Firmicutes, Proteobacteria, Bacteroidota and Actinobacteriota dominated at casing and the end of the crop cycle. The most dominant fungal genus detected at casing was Agaricus and after 7 days accounted for the vast majority of fungal species detected. The application of BCAs, B. velezensis QST 713 and B. velezensis Kos did not have a significant impact on the microbiota across the crop cycle and comparisons between control plots and plots treated with BCA showed no significant differences in their microbiome composition. This research contributes novel insights into the dynamics, composition, and structure of microbial communities within A. bisporus mushroom casing with and without the application of two BCAs.