A wet incipient impregnation procedure was developed to infiltrate Mg(BH4)2 into the voids of pre-treated activated carbon with a pore diameter of < 2 nm. The thermal data of the composite material showed a strong broadening of the signals. The peak decomposition temperature was shifted to lower values by the infiltration. A Kissinger analysis of bulk Mg(BH4)2 and the nanocomposite revealed a high activation barrier for the first step of the dehydrogenation of the bulk material, which was lowered by a factor of two for the nanoconfined hydride.