GOES-16 and GOES-17 are the first of NOAA’s Geostationary Operational Environmental Satellite (GOES)-R series of satellites. Each GOES-R satellite has a magnetometer mounted on the end (outboard) and one part-way down a long boom (inboard). This paper demonstrates the relative accuracy and stability of the measurements on a daily and long-term basis. The GOES-16 and GOES-17 magnetic field observations from 2017 to 2020 have been compared to simultaneous magnetic field observations from each other and from the previous GOES-NOP series satellites (GOES-13, GOES-14 and GOES-15). These comparisons provide assessments of relative accuracy and stability. We use a field model to facilitate the inter-satellite comparisons at different longitudes. GOES-16 inboard and outboard magnetometers data suffer daily variations which cannot be explained by natural phenomena. Long-term averaged GOES-16 outboard (OB) data has daily variations of ± 3 nT which are stable within ± 1.5 nT. Long-term averaged GOES-17OB magnetometer data have minimal daily variations (less than ± 1 nT). Daily average of the difference between the GOES-16 outboard or GOES-17 outboard measurements and the measurements made by another GOES satellite are computed. The long-term averaged results show the GOES-16OB and GOES-17OB measurements have long-term stability (± 2 nT or less) and match measurements from magnetometers on other GOES within limits stated herein. The GOES-17OB operational offset (zero field value) was refined using the GOES-17 satellite rotated 180° about the Earth pointing axis (known as a yaw flip).