The coarsening behavior of solid particles significantly affects the alloy microstructure during semi-solid manufacturing. Notably, high magnetic fields can be used to regulate coarsening, by leveraging magnetic torque. Herein, the effect of high magnetic fields on the coarsening behavior of Al particles in the Al–15 wt% Cu alloy was investigated, considering the dependence on the magnetic flux density (0, 2, 4, 6, 8, and 10 T) and holding time (60, 180, and 300 min). Regardless of the magnetic field, the Al particle size increased with prolonged holding times in the Al–15 wt% Cu alloy; however, the coarsening rate significantly accelerated when a magnetic field was applied. Additionally, the size, connecting neck size, and coordination number of the particles increased with the increasing magnetic flux density. For a higher magnetic flux density, longer holding times led to enhanced particle contact and coalescence, which further accelerated coarsening. Moreover, high magnetic fields promoted a shift of the coarsening mechanism from Ostwald ripening to a combination of Ostwald ripening and migration–coalescence. The accelerated coarsening of the Al particles was attributed to a reduced orientation difference among the particles, due to the rotation and reorientation induced by the magnetic torque. This work revealed the impact of magnetic flux density and holding time on the coarsening behavior of Al particles in an Al–Cu alloy, supporting the development and optimization of magnetic field-assisted semi-solid manufacturing processes.