One of the primary objectives of the Double Star mission is the accurate measurement of the magnetic field vector along the orbits of the two spacecraft. The magnetic field is an essential parameter for the understanding of space plasma processes and is also required for the effective interpretation of data from the other instruments on the spacecraft. We present the design of the magnetic field instrument onboard both of the Double Star spacecraft and an overview of the performance as measured first on-ground and then in-orbit. We also report the results of in-flight calibration of the magnetometers, and the processing methods employed to produce the final data products which are provided to Double Star investigators, and the wider community in general. Particular attention is paid to the techniques developed for removing magnetic interference generated by the solar arrays on the first (equatorial orbiting) spacecraft. Results from the first year of operations are reviewed in the context of combined observations by Double Star and Cluster, and examples given from the different regions visited by the spacecraft to date.