Magnetic ordering of the PbMnTeO6 crystallizing in the chiral non-centrosymmetric P312 space group has been experimentally investigated by high resolution neutron powder diffraction (NPD) and theoretically by high temperature expansion (HTE), density matrix renormalization group (DMRG) and linear spin wave theory (SWT) approaches. Processing of low-temperature NPD data showed that to describe a rather complex spin ordering in PbMnTeO6, two incommensurate modulations determined by the propagation vectors k1 and k2 should be used. The observed ground state with k1 = (1/3 1/3 0.4077) is a non-collinear 120-degree spin structure, which is helically ordered along the layer stacking direction with a characteristic spiral period of ~13.2 Å. Besides, the second incommensurate modulation for magnetic structure with the propagation vector k2 = (1/3+δ1 1/3+δ1 δ2) was also observed. An effective spin model for explaining the observed magnetic characteristics in PbMnTeO6 was also constructed from numerical calculations. The dominant nearest-neighbor interaction was estimated as J_1=12±2K by fits of magnetic susceptibility, magnetization curve, and magnetic part of specific heat between experimental and numerical results. It is shown that spiral ordering along the layers stacking direction can be attributed to frustrated interlayer couplings or the presence of an interlayer Dzyaloshinskii–Moriya interaction. Also, the incommensurate modulation of propagation vector in the ab plane can be reproduced by introducing a subtle anisotropic distortion of triangular lattice.