In this work, AlMgB14-based materials were obtained from an (Al–Mg)–B powder mixture by spark plasma sintering. The microstructure, phase composition, and properties of the obtained materials were investigated. The sample, which was sintered at a temperature of 1400 °C, has the highest average hardness of 29.5 GPa with a maximum measured hardness value of 34.55 GPa. The density of the obtained sample is 2.621 g/cm3 (theoretical density of pure AlMgB14 is 2.59 g/cm3). A mechanism for the decomposition of AlMgB14 at local overheating of the (Al–Mg)–B powder mixture during spark plasma sintering was proposed. In the first stage, AlMgB14 decomposes into AlB12, an intermetallic compound Al12Mg17 and Mg. In the second stage, the decomposition products react with oxygen to form spinel MgAl2O4 and aluminum borate Al4B2O9. Finally, the use of isothermal holding for 5 min at a temperature of 1470 °C leads to an increase in the MgAl2O4 spinel content, further reaction of Al2O3 and boron oxide B2O3 and, as a consequence, the formation of borate Al18B4O33.