To explore the possible occurrence of new stacking sequences within the La-Mg-Ni system, two alloys in the composition range La15.6-16.8Mg6.3-7.8Ni76.5-76.9 (at. %) have been synthesized by Spark Plasma Sintering (SPS). The X-ray diffraction analyses show that the obtained alloys contain a mixture of A2Ni7 and A5Ni19 phases (A = La, Mg) with coexistence of hexagonal and rhombohedral polymorphs. Such structures can be described as a stacking along the c crystallographic axis of mAB5 units (labeled C) and nA2B4 units (labeled L). Up to now, compounds with 1 ≠ m ≠ 4, but only n = 1 were confirmed in the La-Mg-Ni system. One composition has been particularly studied here by High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM). A continuous change of the stacking sequences (from C + L to 4C + L) is observed within a 50 nm region. In addition, two types of stacking faults (twinning and dislocation) have been revealed. The expected stacking sequence with n > 1 has not been observed in the present work. The hydrogen storage capacity and the electrochemical capacity are not significantly affected by the phase compositions, while the cyclic stability appears particularly sensitive to the Mg content.