Efforts to abate climate change heavily rely on carbon sequestration by trees. However, analyses of tree carbon dynamics often neglect trees outside of forests (TOFs) and spatially detailed information about tree carbon sequestration rates are largely missing. Here we describe a new method which combines remote sensing with forest inventory data from 127 358 sites to first estimate tree age and site productivity, which we then used to estimate carbon storage and sequestration rates for all trees inside and outside forests across Great Britain. Our models estimate carbon storage and sequestration rates with R² values of 0.86 and 0.56 (root-mean-square errors of 70 tCO₂e ha^-1 and 3.4 tCO₂e ha^-1 yr^-1). They also reveal the important finding that 17% (165.6 MtCO₂e) of the total carbon storage and 21% (3.4 MtCO₂e yr^-1) of the total carbon sequestration rate of all trees in Great Britain come from TOF, with particularly high contributions in England (24.3% and 34.1%), followed by Wales (12.5% and 17.6%) and Scotland (2.6% and 1.8%). Future estimates of carbon status and fluxes need to account for the significant contributions of TOF because these trees, often found in field margins and hedgerows are potentially an important carbon offset. Our novel approach enables carbon baseline assessments against which changes can be assessed at management relevant scales, improving the means to measure progress towards net zero emissions targets and associated environmental policies.