The reaction of MgCl2, NiCl2, and Na2(i-mnt) (i-mnt is 1,1-dicyanothene-2,2-dithiolate) with 15-crown-5 (15-C-5) leads to an infinite chain polymer, {[NiMg2(C4N2S2)2(C10H20O5)2]Cl2}n or {[Mg(15-C-5)]2[Ni(i-mnt)2]Cl2}n, which consists of two [Mg(15-C-5)]2+ complex cations, one [Ni(i-mnt)2]2− complex anion and two Cl− ions per formula unit. In the [Ni(i-mnt)2]2− complex anion, Ni2+ is located on a crystallographic mirror plane with a slightly distorted square-planar coordination by four S atoms. In the [Mg(15-C-5)]2+ complex cations, the Mg and one O atom of the crown lie on mirror planes and the Mg atoms are in sevenfold coordination environments of five O atoms from the crown and two N atoms from two i-mnt anions. The bridging of the two complexes via the Mg—N bonds leads to the formation of one-dimensional chains along the a axis.