Before reaching the Jovian system in 2031, the ESA Jupiter Icy Moons Explorer (Juice) spacecraft will perform three Earth flybys. As well as providing gravitational assistance, these flybys are prime opportunities to assess performance and undertake in-flight calibration of the science payload in a well-understood environment. The first such interval occurred in August 2024, dubbed the Lunar-Earth gravity assist (LEGA) as it included a pass by the Moon in advance of the approach to Earth. The unique spacecraft trajectory allowed for sampling of various regions of the magnetosphere before exiting into the solar wind. In this paper, we report on the performance of the Juice magnetometer instrument (J-MAG) during the LEGA. J-MAG comprises three sensors—two fluxgate vector sensors and one Coupled Dark State Magnetometer (CDSM) scalar sensor—all of which were operational during the LEGA period. This represents the first time in the mission that the scalar sensor has operated in its nominal state. Here, we analyse J-MAG observations during key periods of the flyby. As well as inter-sensor comparison, we assess J-MAG data against geomagnetic field models during the approach to Earth, and compare with measurements from other spacecraft at the Moon and in the solar wind, which allow us to make suggestions for future calibration activities. Overall, the three sensors showed excellent performance at this early stage in the mission and confirmed that the scalar sensor meets the requirements for in-flight calibration of the fluxgate sensors at Ganymede. The LEGA also demonstrates the potential value of using cruise phase measurements for scientific exploration and solar wind monitoring.