This dataset contains monthly mean magnetic data from the Greenwich Observatory from 1841 to 1925. This dataset is the result of report https://nora.nerc.ac.uk/id/eprint/536965/ and is the published product of a study by the British Geological Survey (BGS). The main aim was to create a digital record of the monthly magnetic data stored in the Greenwich Magnetic Observatory yearbooks for 1841 to 1925, for the magnetic research community to use. The report also details the steps taken to convert the inclination and declination, from various published units to standardised units (arcminutes). We have also attempted to convert the horizontal and vertical force values published from 1868 to 1911 from British, and Gauss units to nanoTeslas, however, due to inconsistencies in the scaling factors, and time constraints, we have had limited success. Post-1912 the horizontal and vertical force data has successfully been converted into nanoTesla. Greenwich Magnetic Observatory (GRW) is located at 51°29'N 0°0'E 0m elevation. Observatory info can be found at: https://geomag.bgs.ac.uk/operations/greenwich.html The observatory yearbooks can be found at: http://www.geomag.bgs.ac.uk/data_service/data/yearbooks/grw.html A report describing this data set and its construction can be found at: https://nora.nerc.ac.uk/id/eprint/536965/