To explore the effects of a co-ligand on the construction of mixed-ligand metal–organic frameworks (MOFs), two new triazole-based complexes with a flexible dicarboxylate as a co-ligand, {[Zn₄(trz)₄(gt)₂(H₂O)₂](H₂O)₂}_n 1 and {[Cd₂(trz)₂(gt)(H₂O)₂](H₂O)₄}_n 2 (Htrz = 1,2,4-triazole; H₂gt = glutaric acid), were synthesized and their structures were fully characterized by elemental analyses, IR spectroscopy, and single-crystal X-ray crystallography. Their thermal stability and luminescence emissions were further investigated to establish their structure–property relationship. Crystal structure determination showed that 1 is a neutral two-dimensional pillared-bilayer network consisting of 14-membered hydrophobic channels, whereas 2 is an infinite three-dimensional framework constructed from tetranuclear [Cd₄(trz)₄]⁴⁺ subunits. Interestingly, the overall structure of both MOFs can be solely supported by Zn^II/Cd^II and trz anions, and were further consolidated by the introduction of a flexible gt co-ligand. In addition, the carboxylate groups in the co-ligand can also serve as a weak O–H···O hydrogen-bond acceptor to capture guest water molecules. The synchronous weight-loss behaviour of trz and gt anions presented by thermogravometric curves suggest their cooperative contributions to the thermal stability of the MOFs. In contrast, the fluorescence emissions of two complexes are significantly dominated by the core trz ligand, rather than the gt co-ligand and metal ions.