Iron oxide nanoparticles have garnered significant interest in recent years due to their diverse applications, particularly in the therapeutic field. We present a green synthesis method using the extract of Senna bicapsularis, the production of iron oxide nanoparticles (IONPs). The successful synthesis of IONPs was confirmed by UV–visible spectroscopy, revealing the characteristic peak at 295 nm. Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy were employed to elucidate the functional groups involved in the synthesis and characterize the morphological features of the nanoparticles. Subsequently, the synthesized IONPs were subjected to biological assays to assess their anticancer, enzyme inhibitory, analgesic, and sedative activities, following standardized protocols. The IONPs exhibited potent anticancer activity against the MDR 2780AD cell line, with IC50 values of 0.85 (extract) and 0.55 (iron oxide nanoparticles). Remarkable inhibitory effects were also observed against urease (IC50 = 12.98 ± 0.98) and xanthine oxidase (IC50 = 96.09 ± 0.65). Additionally, they demonstrated moderate carbonic anhydrase II inhibition, with 42.09% inhibition at a concentration of 0.25 mM. Furthermore, the extract and IONPs demonstrated a significant analgesic effect in a dose-dependent manner, while the sedative effect was also significant (p < 0.001).