A phenomenological analysis of the magnetic and structural phase states observed in the manganese arsenide-based alloys over a wide range of temperatures and pressures has been performed. The unique first-order phase transitions from the paramagnetic and metamagnetic states to the ferromagnetic state are studied experimentally. It is found that these transitions exhibit characteristic features associated with the magnetostriction suppression of nucleation when the lability boundary of the paramagnetic phase is absent and the transition from the metamagnetic state is not thermodynamically "forbidden." The phase behavior of manganese arsenide-based alloys is interpreted within the proposed model. This model accounts for the interrelation of the magnetic, structural, and elastic properties of alloys and also the specific features of phase transformations in real crystals.