Bio-synthesis of nanostructures is an ever green approach; therefore, the current work attempts to use Arabic gum as bio-synthesizer for manufacturing Ag–Au bimetallic nanocomposite, using seed-mediated growth technique. The relevance of the as-used natural gums for manufacturing of Ag–Au nanocomposite in quite simple, energy saving and cost effective methodology, has been successfully approved. Arabic gum acted concurrently as a reducer for metal ions to nano-sized structures and crystal growth modifier of the so-generated nanocomposite. Monitoring of interchanging in size distribution of the as-produced bimetallic nanocomposite by the effect of the gum concentration, concentration of nanometals precursors, their addition sequencing in reaction liquor, and reaction temperature, was illustrated and well characterized. UV–Visible spectra showed that the characteristic surface plasmon resonance peak for Ag–Au bimetallic nanocomposite was observed at 480–495 nm and XRD patterns confirmed the preparation of bimetallic nanostructures. According to zetasizer analyses and TEM micrographs, small sized (3.1 nm) Ag–Au bimetallic nanocomposite with narrow size distribution (1–7 nm) were obtained. The all prepared bimetallic nanocomposite showed good stability with polydispersity index ranged in 0.203–0.530. The mechanism of nanostructures’ generation was suggested and approved using FT-IR, 1HNMR and 13CNMR spectra. The prepared Ag–Au bimetallic nanocomposites showed excellent antibacterial activity against both of gram + ve and gram − ve strains.