The magnetic phase diagram of the Mn-based semi-Heusler alloys is determined at T=0 using first-principles calculations in conjunction with the frozen-magnon approximation. The authors show that the magnetism in these systems strongly depends on the number of conduction electrons, their spin polarization, and the position of the unoccupied Mn 3d states with respect to Fermi energy. Various magnetic phases are obtained depending on these characteristics. The conditions leading to diverse magnetic behavior are identified. The obtained results are in very good agreement with the available experimental data.