Several borohydridolanthanidocenes were synthesized by means of a straightforward approach starting from equimolar amounts of Ln(BH4)3(thf)3 (Ln = Nd, Sm) and n‐butylethylmagnesium (BEM) in the presence of 2 equiv. of a cyclopentadiene derivative. Depending on the nature of the cyclopentadiene, monometallic Cp*2Nd(BH4)(thf) (Cp* = C5Me5) (1), heterodimetallic [Cp2Nd(BH4)(μ‐BH4)]2Mg(thf)4 (Cp = C5H5) (2), [(CMe2C5H4)2Ln(BH4)(μ‐BH4)]2Mg(thf)3 (Ln = Nd, 3a; Ln = Sm, 3b), and monometallic anilidocyclopentadienyl compounds (C5Me4CH2SiMe2NPh)Ln(BH4)(thf)2 (Ln = Nd, 4a; Ln = Sm, 4b) were isolated. The complexes were characterized by 1H NMR spectroscopy, elemental analysis, and X‐ray structural analysis. Mg(BH4)2(thf)3 (5) was isolated as a byproduct and structurally characterized, revealing an unprecedented structural arrangement. In combination with BEM, complexes 2a, 3a, and 4a were found to be efficient in isoprene polymerization to afford highly stereoregular trans‐polyisoprene. Similar results were obtained by an “all in situ” approach starting from Nd(BH4)3(thf)3/BEM mixtures in the presence of 2 equiv. of cyclopentadiene derivative.