Stimulated by recent experimental observations of room-temperature ferromagnetism of MnxCd1-xGeP2 and MnxZn1-xGeP2, we investigate the structural, electronic, and magnetic properties of this class of systems (II-Ge-V-2, II=Zn, Cd, and V=As, P) as a function of Mn concentration and chemical constituents by means of first-principles density-functional-theory-based codes. Our calculations indicate that, for Mn substituting the II element, the anti ferromagnetic alignment is the most stable ordering for all the systems studied. For Zn- and Cd-rich systems, the total magnetic moments per Mn atom of the ferromagnetic phase is very close to the ideal value of 5 muB, since the Mn 3d states in the minority spin channel are nearly empty; on the other hand, for Mn rich compounds, the stronger p-d hybridization lowers the total magnetic moment to about 4.4 muB.