Microstructural evolutions and grain-boundary-character distribution during high-energy-beam welding of ultra-thin FeCo-V foils were studied. Detailed data about the boundaries, coincidence site lattice (CSL) relationships, grain sizes, and microstructural features were acquired from electron-backscatter diffraction (EBSD) maps. Moreover, the evolution of the magnetic properties during high-energy-beam welding was studied using vibrating sample magnetometry (VSM). The fraction of low-angle boundaries was observed to increase in the fusion zones of both electron- and laser-beam-welded foils. The results showed that the fractions of low-Σ CSL boundaries (particularly twin boundaries, Σ3) in the fusion zones of the welded foils are higher than those in the base metal. Because the strain rates produced during high-energy-beam welding are very high (because of the extremely high cooling rate), grain deformation by a slip mechanism is limited; therefore, deformation by grain twinning is dominant. VSM analysis showed that the magnetic properties of the welded foils, i.e., their remanence, coercive force, and energy product, changed significantly. The formation of large grains with preferred orientation parallel to the easy axis of magnetization was the main reason for the diminished magnetic properties.