We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in $ACo_{2}As_{2}$ (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of $EuCo_{2}As_{2}$ exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of $CaCo_{2}As_{2}$, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-$EuCo_{2}As_{2}$, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in $Ca_{0.9}Eu_{0.1}Co_{1.91}As_{2}$ or direct electron doping in $Ca_{0.85}La_{0.15}Co_{1.89}As_{2}$. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material’s properties via involvement of strongly correlated electrons.