Two novel coumarin-based ligands, coumarin-6,7-dioxyacetic acid (1) (cdoaH2) and 4-methylcoumarin-6,7-dioxyacetic acid (2) (4-MecdoaH2), were reacted with copper(II) and manganese(II) salts to give [Cu(cdoa)(H2O)2] · 1.5H2O (3), [Cu(4-Mecdoa)(H2O)2] (4), [Mn(cdoa)(H2O)2] (5) and [Mn(4-Mecdoa)(H2O)2] · 0.5H2O (6). The metal complexes, 3–6, were characterised by elemental analysis, IR and UV–Vis spectroscopy, and magnetic susceptibility measurements and were assigned a polymeric structure. 1 and 2 react with Cu(II) in the presence of excess 1,10-phenanthroline (phen) giving [Cu(cdoa)(phen)2] · 8.8H2O (7) and [Cu(4-Mecdoa)(phen)2] · 13H2O (8), respectively. The X-ray crystal structures of 7 and 8 confirmed trigonal bipyramidal geometries, with the metals bonded to the four nitrogen atoms of the two chelating phen molecules and to a single carboxylate oxygen of the dicarboxylate ligand. The complexes were screened for their antimicrobial activity against a number of microbial species, including methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli and Candida albicans. The metal-free ligands 1 and 2 were active against all of the microbes. Complexes 3–6 demonstrated no significant activity whilst the phen adducts 7 and 8 were active against MRSA (MIC80 = 12.1 μM), E. coli (MIC80 = 14.9 μM) and Patonea agglumerans (MIC80 = 12.6 μM). Complex 7 also demonstrated anti-Candida activity (MIC80 = 22 μM) comparable to that of the commercially available antifungal agent ketoconazole (MIC80 = 25 μM).