A novel series of coumarin-4-carboxylate ligands (2a–2l), their Ag(I) complexes (3a–3l) together with a number of their phenanthroline adducts (4d, 4f and 4g) have been synthesised and characterised. The ligands were prepared by either acid or base hydrolysis of their corresponding esters or by demethylation of a methylated acid derivative. The esters (1a–1i) were synthesised by an aromatic electrophilic substitution reaction between the conjugate base of substituted phenols and a vinyltriphenylphosphonium salt. The novel series of Ag(I) carboxylate complexes were prepared by reaction of silver nitrate with the coumarin ligands in aqueous/alcohol solution. The Ag(I) phenanthroline adducts were prepared by reaction of the Ag(I) complex with 1,10-phenanthroline in aqueous/alcohol solution. A selection of complexes were screened for their in vitro antibacterial activity against Pseudomonas aeruginosa grown planktonically or as a biofilm as well as a number of other clinically important strains, Escherichia coli, MRSA and Staphylococcus aureus. The activity against the P. aeruginosa biofilm was found to be significantly greater than against the planktonic bacteria.