The crystal structure of the ternary germanide Li2MnGe has been re-evaluated from single-crystal X-ray diffraction data. This compound crystallizes in a non-centrosymmetric superstructure of the ZrCuSiAs type (space group P4bm, Pearson code tP16), with the lattice parameters a = 6.088(4) angstrom, c = 6.323(4) angstrom. First-principle calculations for the idealized structure predict antiferromagnetic exchange in the square Mn nets and semimetallic ground state. In addition, a new ternary phase with the composition Li2-xMn4+xGe5 (x approximate to 1.2) was discovered. It adopts the V6Si5 structure type (space group Ibam, Pearson code oI44), with the lattice parameters a = 7.570(2) angstrom, b = 16.323(3) angstrom, c = 5.057(1) angstrom. DSC/TG measurements show that this compound is thermally stable below 995 K.