This dataset contains dissolved methane concentrations from groundwater in Great Britain, predominantly from England. Data were collated in Microsoft Excel (.xlsx) format from four publicly accessible sources: the British Geological Survey (from Bell et al., 2017), the Environment Agency for England (from the online Water Quality Archive), and local baseline data for the shale gas sites of Preston New Road (from Cuadrilla Resources) and Kirby Misperton (from Third Energy). In total, 2997 dissolved methane concentrations are included in the dataset and were collected from 922 unique locations from October 1992 to July 2018. The data were used within a Bayesian framework to create a dynamic baseline (a baseline that can change in time and space to reflect ongoing environmental change) that can predict the probability that a change in dissolved methane concentration has occurred, for example in the context of monitoring aquifers at shale gas sites using hydraulic fracturing methods. The data presented are the underlying dataset to Wilson et al., 2020, Science of the Total Environment: https://doi.org/10.1016/j.scitotenv.2019.134854. Bell, R. A., Darling, W. G., Ward, R. S., Basava-Reddi, L., Halwa, L., Manamsa, K., & Dochartaigh, B. Ó. (2017). A baseline survey of dissolved methane in aquifers of Great Britain. Science of the Total Environment, 601, 1803-1813. Wilson, M. P., Worrall, F., Davies, R. J., & Hart, A. (2020). A dynamic baseline for dissolved methane in English groundwater. Science of The Total Environment, 711, 134854.