The redistribution of AlB₂ and AlB₁₂ reinforcing particles in functionally graded Al–B–Mg composites was investigated as a function of centrifugal casting time, rotational speed, and pouring temperature of the melt. The microstructure was characterized both qualitatively and quantitatively on four different sections along the main axis (centrifugal force direction) of the cylindrical casting. The resulting aluminum matrix composite showed an evident gradient of particles concentration, with more particles at the external zone than at the internal zone of the casting. Processing temperature was found critical for its direct effect on the viscosity of the liquid, which regulates the segregation of the reinforcements. As expected, the higher concentration of particles on the external regions of the castings raised the measured superficial Rockwell hardness and Vickers microhardness of the composite. Finally, complementary energy dispersive spectroscopic analysis indicated that Mg dissolved in the AlB₂ dispersoids but not in the AlB₁₂ particles.