The development of green method of metal nanoparticles (NPs) synthesis has become a major focus of researchers as the biogenic reduction of metals into NPs is eco-friendly, less expensive and free of hazardous chemicals. In the present study, we report the synthesis and characterization of gold (Au) NPs using aqueous Cassia fistula leaf extract. The reduction of 1 mM HAuCl4 (chloroauric acid) occurred within 7 min after addition of the leaf extract. These Au NPs were further characterized by using UV-visible spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR) analysis. MTT assay was carried out to check cytotoxicity of these Au NPs on MCF-7 cells. UV-Vis spectra confirmed the synthesis of Au NPs with SPR band obtained at 535 nm. The XRD analysis confirmed the crystalline nature of Au NPs. The average size of 27.21 nm was obtained by TEM analysis. The FTIR spectra explained the role of plant phyto-constituents in reduction and stabilization of Au NPs. We finally propose a reduction mechanism using HR-LCMS analysis of Cassia leaf extract showing the presence of flavonoids, sugar and antioxidants as supported by FTIR and NMR results. These bioreduced Au NPs showed no significant cytotoxicity to MCF-7 cells.