A series of bis-thiosemicarbazone agents with coordinating groups capable of multiple metal coordination modes has been generated and evaluated for potential cytotoxic effects against melanoma (MelRm) and breast adenocarcinoma (MCF-7) cell lines. The bis-thiosemicarbazones in this study generally demonstrated superior cytotoxic activity against MelRm than MCF-7 in the absence of metal ion supplementation, but in most cases could not be considered superior to the reference thiosemicarbazone Dp44mT. The key structural features for the cytotoxic activity were the central metal binding atom on the aromatic core, the thiocarbonyl residue and the nature of substitution on the N4-terminus in terms of size and lipophilicity. The cytotoxicity of bis-thiosemicarbazone ligands improved significantly with Cu(II) supplementation, particularly against MCF-7 cells. The mechanism of cytotoxicity of bis-thiosemicarbazones was proposed to be dependent on the combined effect of metal mobilisation and ROS generation which is so called a 'double-punch effect'.