Geochemical measurements which quantify the lithium isotope ratios (d7Li) of Paleozoic (541–251 Mya) mudstones. Samples were obtained from various field locations across Scotland, Wales, England and New Brunswick, Nova Scotia, Canada. Sampled mudstones are listed under their formation name, with information on the locations of each outcrop belt and further details on lithological characteristics, including environment of formation, freely available on the British Geological Survey Lexicon of named rock units (https://www.bgs.ac.uk/technologies/the-bgs-lexicon-of-named-rock-units/) and Government of Canada weblex (https://weblex.canada.ca/weblexnet4/weblex_e.aspx), for UK and Canadian samples, respectively. Stratigraphic age is given in accordance to the GSA geological timescale v.5.0. Following sampling, specific methodologies for preparation for Lithium isotope analysis are provided in the Metadata Lineage. The data was collected to understand changes in weathering intensity coeval with the Paleozoic expansion of land plants, with lithium isotopes a powerful trace for silicate weathering as they are sensitive to the balance between rock dissolution and clay formation. The tabulated lithium isotope ratios were compared at different temporal stages of plant evolution through the Paleozoic. Samples were collected by the University of Cambridge. Lithium isotope ratios were obtained by William McMahon and supervised by Edward Tipper and Mohd Tarique. Mass spectrometry was carried out by William McMahon and David Wilson at University College London.