In this work we describe the preparation, spectroscopic, thermal and structural characterization, and cryomagnetic investigation of four new polymorphic derivatives of the [M2(H2mpba)3]2– mesocate of formulas {[K2(dmf)2(H2O)2]­[M2(H2mpba)3]·2H2O} n (M = Co (1), Ni (2)) and [M­(tppz)2]­[M2(H2mpba)3]·9H2O (M = Co (3), Ni (4)) (H4mpba = 1,3-phenylenebis­(oxamic) acid), tppz = 2,3,5,6-tetrakis­(2-pyridyl)­pyrazine). Pairs 1/2 and 3/4 are isomorphous compounds exhibiting a triple-stranded dinuclear motif, [M2(H2mpba)3]2–. 1 and 2 exhibit a three-dimensional network with [M2(H2mpba)3]2– mesocates interlinked by potassium­(I) ions. 3 and 4 present a brick and mortar network, where the “brick” is [M2(H2mpba)3]2– + [M­(tppz)2]2+ and the “mortar” refers to water molecules, where the electroneutrality of the motif is achieved by the [M­(tppz)2]2+ cationic complex. The dc magnetic data for 1–4 in the temperature range 1.9–300 K reveal the occurrence of magnetically isolated high-spin cobalt­(II) of the mesocate units in 1 and 3 plus a thermally induced spin crossover behavior of the [Co­(tppz)2]2+ unit in 3, whereas 2 and 4 exhibit Curie law behavior. Out-of-phase ac signals are observed for 1 and 3 at temperatures below 14 K under zero and nonzero applied dc fields, which are characteristic of the occurrence of slow magnetic relaxation in these systems, thus being new examples of single-molecule magnets (SMMs).