Silver nanoparticles (AgNPs) were synthesized via a rapid, one-pot green route using Ophiopogon japonicus aqueous extract (OJAE) as both reducing and stabilizing agent, eliminating the need for toxic chemicals. Optimal synthesis conditions (200 µL OJAE, 60°C, 30 min, pH 12, 0.5 mM AgNO3) were established using a one-factor-at-a-time (OFAT) strategy. UV–vis spectroscopy confirmed AgNPs formation with a distinct surface plasmon resonance peak at 407 nm. The resulting AgNPs were predominantly spherical (15.5 ± 3.0 nm), highly crystalline (d-spacing: 0.238 and 0.256 nm), and stable (zeta potential: −26.7 mV). They exhibited strong antibacterial activity, with inhibition zones against Staphylococcus aureus and Escherichia coli exceeding 98%, comparable to standard antibiotics. Furthermore, the AgNPs achieved > 99% catalytic decolorization of acid blue 29 and acid red 1 within 20 and 6 min, respectively, following pseudo-first-order kinetics (k = 0.2930 ± 0.0189 and 0.9843 ± 0.0873 min−1; r2 = 0.9754 and 0.9844). These results establish OJAE as an effective biogenic precursor for producing multifunctional AgNPs with promising applications in antimicrobial therapy and textile wastewater remediation.