Biosynthesis and characterizations of nanoparticles has become an important branch of nanotechnology nowadays. In this paper, green synthesis of silver nanoparticles (AgNPs) using the alcoholic extract of Argemone mexicana Linn. as a reducing and stabilizing agent, has been discussed. This biosynthetic method is simple, cost-effective and reproducible. Formation of AgNPs was established by X-ray diffraction, transmission electron microscopy and UV-Visible spectroscopic techniques. Nanoparticles almost spherical in shape having a size of 2-6 nm are found. UV-visible study revealed the surface plasmon resonance at 414 nm. A possible involved mechanism for the biosynthesis of silver nanoparticles has also been proposed.  Further, it was found that AgNPs sol when applied to the SiHa cancer cell line was found to inhibit the growth by 70-80%. It is cumulative effect of the unutilized plant extract and nanosilver. The work signifies the importance of medicinal plants in synthesis of nanomaterials as it bestows double benefit in terms of drug delivery as well as safety. It may open a fresh avenue in future cancer therapeutics.