New materials for the next generation of electrochemical energy storage devices such as batteries are of extreme importance. Here we investigate the structure, ionic conductivity and thermal properties of isopropylamine magnesium borohydride based composites with different compositions, Mg(BH4)2·x(CH3)2CHNH2, x = 0.5, 0.9, 1.25, 1.5, 1.75, 2.5, 3.1. Three new compounds are discovered, x = 1, 2, and 3 and the monoclinic structure of Mg(BH4)2·2(CH3)2CHNH2 (P21/c) is investigated in detail. This structure consists of neutral complexes [Mg(BH4)2((CH3)2CHNH2)2] di-hydrogen bonded to form layers and these layers are connected by hydrophobic interactions via the isopropyl moieties. The orthorhombic unit cell of Mg(BH4)2·(CH3)2CHNH2 was also determined, a = 9.78, b = 12.17 and c = 17.24 Å. In general, the samples are thermally stable up to 50 °C where they started to become softer, and at 70 °C isopropylamine release and melting started. The highest Mg2+ ionic conductivity was that of Mg(BH4)2·1.5(CH3)2CHNH2, σ(Mg2+) = 2.7 × 10-4 S cm-1 at 45 °C, with an activation energy of EA = 1.22 eV. Furthermore, reversible stripping/plating of Mg was displayed at 45 °C, with an oxidative stability of 1.2 V vs. Mg/Mg2+. The addition of MgO nanoparticles (75 wt%) improves the mechanical and thermal stability, and decreases the activation energy, to EA = 0.56 eV. Thereby the Mg2+ conductivity is increased at low temperature. This suggests that the hydrophobic interactions contribute to the high ionic conductivity in the solid state, which opens a new avenue for design and discovery of electrolyte materials.