The two compounds LaMn₂Ge₂ and LaMn₂ Si₂ exhibit different noncollinear and collinear magnetic structures and temperature-driven transitions between them. A phenomenological study of the behaviour of each of them requires a Hamiltonian containing the interactions between the localized moments. We assume a model Hamiltonian with bilinear, biquadratic and anisotropic interactions for which the interaction constants are taken from ab initio calculations and evaluate the free energies of the above-mentioned configurations within a mean field approximation (MFA). The experimentally observed phase sequences are obtained for both compounds.