Cadmium sulphide nanoparticles (CdS NPs), renowned for their tunable bandgap, exhibit significant potential for photo chemical applications. A simple, green, cost-effective synthesis approach is quite desirable to harness their full potential. With this prospect, we present the green synthesis of CdS NPs using cinnamon extract as a reducing and stabilizing agent. The as-synthesized NPs were characterized with the aid of diverse characterization techniques. The optical properties and bandgap were performed using a UV–Vis spectrophotometer, which was found to be 2.91 eV. The X-ray diffraction (XRD) revealed the crystalline nature of NPs and an average crystallite size of 5 nm. The morphology and shape were analyzed by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). The surface of synthesized NPs was found to be spherical, and some irregular-shaped NPs were observed. The bioengineered CdS NPs showed a pH of the zero charge point charge (pHZPC) of approximately 6.5 representing the NP surface charge as neutral. Further, the semiconductor properties were evaluated using photoelectrochemical (PEC) studies, demonstrating charge transport dynamics. CdS NPs exhibited efficient photocatalytic degradation of MB dye: 91% in 90 min under UV light and 94% in 70 min under sunlight. The cytotoxicity of the CdS NPs was performed against MCF-7 breast cancer cells with an IC50 value of 110 µg/mL, significantly lower than the IC50 of cisplatin which is 31.63 µg/mL. In the biological applications, CdS NPs exhibited a slight haemolytic nature at high concentrations demonstrating their biocompatibility. In the antibacte rial activity of NPs, the results are moderately good. This comprehensive investigation underscores the potential as efficient photocatalysts for dye degradation and exhibiting cytotoxicity against breast cancer cells.