The mechanical properties of soft tissue have long been of interest in biomedical research and applications, and increasingly for breast cancer imaging. In this paper, the mechanical properties of three different materials used to emulate the mechanical behavior of real breast tissue are measured: agar, gelatine, and silicone, to assess their suitability for use in phantoms in systems assessing tissue mechanics for diagnostics. Two widely recognised measurement procedures are used. Quasi-static uniaxial compression was performed under a strain rate of 0.5 mm/min up to 15% strain with preloads of 0.05 N, 0.1 N, and 0.2 N, was used to measure the elastic moduli. Dynamic testing over a frequency range of 0.1-50 Hz for agar and 0.1-100 Hz for gelatine and silicone with the same preload was used to measure the storage moduli. Elastic and storage moduli were (5-81 kPa, 17-85 kPa, 5-112 kPa) and (3-128 kPa, 10-109 kPa, 5-73 kPa) for agar, gelatine, and silicone, respectively at the three preloads. All materials can be cast into arbitrary shapes and are suitable for tissue-mimicking phantoms. Silicone was the most consistent across the different preloads and frequencies, and can provide a range of stiffness ratios of adipose to tumor tissue that match experimentally reported values. More specifically, silicone samples for skin, adipose and tumour tissues show nonlinear stress–strain characteristics at 3 preloads characterized using hyperelastic parameters by fitting NeoHookean, Mooney Rivlin and Ogden models. Silicone also does not contain water, so environmental influences do not affect its mechanical properties as much as the other materials, and is thus more durable for consistent re-use. Finally, breast shaped mimicking silicone phantoms were fabricated for in vitro trials of a Digital Image Elasto Tomography breast cancer screening system assessing changes in mechanical properties.