Atomic and/or vacancy ordering has proven to be important in understanding some unexpected properties in perovskites, leading to the formulation of double perovskites (oxides or halides) as a subclass of these compounds. Intermetallic double perovskite borides (IDPBs) have been described structurally, but the study of their physical properties is still in its infancy. In this work, we have investigated the mechanical hardness and microstructure of the series Sc2Ir6–x Pd x B for the first time. We found that the hardness decreases until x = 5, and then it levels off. This transition coincides with the change in structure from ordered to disordered. A detailed microstructure analysis reveals that the order–disorder transition at x = 5 is accompanied by the precipitation of small amounts of a boron-rich side phase. Interestingly, the hardness trend in the IDPBs (x = 0–4) is accurately reproduced by crystal orbital Hamilton populations (COHP) analysis, especially the total integrated COHP (iCOHP) which measures the bond strength in each compound. Furthermore, it is found that more antibonding states are occupied if Ir is replaced by alioelectronic Pd, thereby reducing the total iCOHP and thus explaining the reduced hardness with increasing Pd concentration.