Whereas carbon isotope ratios of marine carbonates are well characterized through the Phanerozoic, the carbon isotope ratios of terrestrial plant organic matter (δ13Cp) are less so due to the variety of plant parts and materials in which carbon is preserved and the challenges involved in interpreting carbon isotope fractionation in deep time. In 2016, an initial effort to compile published terrestrial organic carbon isotope data for the Phanerozoic (δ13Cp; ISOORG16; Nordt et al. 2016) was accomplished, consisting of 6888 δ13Cp estimates derived from ten organic matter sources. Here, we present an updated version of the Cenozoic and Mesozoic portions of that database (ISOORG23), consisting of 20,334 data points with an expanded 12 groups of organic material (i.e., amber, biomarkers, bulk sediment, coal, copal, leaves, n-alkanes, n-alkanoic acids, paleosol organic matter, peat, wood, and other plant parts). In addition, the entire database was updated to align with the 2020 Geol. Timescale (GTS-2020), with most δ13Cp data points (19,313) placed in 5 Myr bins and hypothetical numerical ages assigned to the subset of 12,828 data points where dating methods were available. Not surprisingly, we found that most published data are from Cenozoic age materials. Mean δ13Cp values for plant material (excluding amber and C3 plants) showed overall depletion in δ13Cp from the Mesozoic (-24.4 ± 0.02‰; n = 7895) to Cenozoic (-25.9 ± 0.02‰ n = 10,230). Among the different organic materials analyzed in ISOORG23, amber showed the highest δ13Cp values (-23.1‰), compared to sub-modern peat and copal with the lowest values (-26.8 and - 27.5‰, resp.). LOESS analyses of the ISOORG23 database with assigned numerical ages were compared to similar analyses performed on a compilation of carbon isotopes of marine calcium carbonate (δ13Ccarb) included in GTS-2020, as well as earlier compilations of atm. CO2 δ13C (δ13CCO2) for the Cenozoic and the Cretaceous portion of the Mesozoic. Those comparisons reveal both periods of correspondence and divergence between δ13Cp and δ13CCO2 and δ13Ccarb values, potentially indicating that major carbon cycle and/or climatic events are present in the δ13Cp data, though interpretation is affected by low data d. during specific periods. A key finding is a consistently increased discrimination in both δ13Cp and δ13CCO2 records that begins at the Miocene Climatic Optimum and continues to the present. However, from the combined Cenozoic and Mesozoic data, that decline in δ13Cp began as early as 135 Ma.