Rare earth metal borohydrides show a number of interesting properties, e.g., Li ion conductivity and luminescence, and the series of materials is well explored. However, previous attempts to obtain M(BH4)3 (M = La, Ce) by reacting MCl3 and LiBH4 yielded LiM(BH4)3Cl. Here, a synthetic approach is presented, which allows the isolation of M(BH4)3 (M = La, Ce) via formation of intermediate complexes with dimethyl sulfide. The cubic c-Ce(BH4)3 (Fm3̅c) is isostructural to high-temperature polymorphs of A(BH4)3 (A = Y, Sm, Er, Yb) borohydrides. The larger size of the Ce3+ ion makes the empty void in the open ReO3-type framework structure potentially accessible to small guest molecules like H2. Another new rhombohedral polymorph, r-M(BH4)3 (M = La, Ce), is a closed form of the framework, prone to stacking faults. The new compounds M(BH4)3 (M = La, Ce) can be combined with LiCl in an addition reaction to form LiM(BH4)3Cl also known as Li4[M4(BH4)12Cl4]; the latter contains the unique tetranuclear cluster [M4(BH4)12Cl4]4- and shows high Li-ion conductivity. This reaction pathway opens a way to synthesize a series of A4[M4(BH4)12X4] (M = La, Ce) compounds with different anions (X) and metal ions (A) and potentially high ion conductivity.