The advancement of nanotechnology has led to the experimental development of cancer therapeutics, which may overcome the shortcomings of commercially available drugs and facilitate improved clinical outcomes. Recently, several metal nanoparticles, especially silver, have been evaluated by scientists globally as useful chemotherapeutic agents due to their multi-functionality and well-recognized biological activity. Herein, we developed silver nitroprusside nanoparticles (abbreviated as AgNNPs) with slight modifications in the reaction conditions and demonstrated their application for breast cancer therapy using in vitro assays and in vivo experiments in a mouse model. Initially, the modified AgNNPs were thoroughly characterized using several analytical techniques. AgNNPs were found to be biocompatible according to in vitro experiments in normal cell lines (HEK-293 and EA.hy926), which was further validated by a hemolysis assay (ex vivo experiment) using mouse red blood cells. In contrast, the cell viability assay using the MTT reagent showed the cytotoxic nature of the AgNNPs against several cancer cell lines (MDA-MB-231, 4T1, B16F10, and PANC-1). Their detailed anticancer activity was investigated using 4T1 (mouse specific) and MDA-MB-231 (human specific) cells through various in vitro assays. The nanoparticles inhibited the formation of blood vessels in the chick embryo model, highlighting their anti-angiogenic behavior. Furthermore, the administration of AgNNPs significantly inhibited orthotopic breast tumor growth (4T1; BALB/c mice) and increased the survivability of the tumor-bearing mice. Also, we demonstrated the plausible molecular mechanisms for the anti-cancer activity of AgNNPs through various in vitro assays and in vivo experiments. Overall, the results support that AgNNPs can be used as an alternative generalized nanomedicine for the treatment of breast and other cancers after proper biosafety evaluation in near future.