Background: Medicinal plants are abundant reservoirs of antimicrobial compounds. Medicinal plants, which are rich sources of many possible medications, are used therapeutically by people in different countries. Justicia adhatoda, a widespread perennial shrub in the tropical region of Southeast Asia, is known for expectorant and antispasmodic activity. In the present study, various extracts of Justicia adhatoda leaves and extract-mediated silver nanoparticles (AgNPs) were assessed for antimicrobial potency against a panel of thirteen multi-drug-resistant bacterial strains and seven fungal strains. Methods: Various solvent extracts of Justicia adhatoda leaves and extract-mediated AgNPs were prepared and dried. Following incubation, extracts were obtained through the evaporation of solvent. The extracts were also evaluated for various parameters such as moisture contents, extractive value, phytochemical and Fourier transform infrared (FT-IR) analysis. The AgNPs were characterized for size and structure using X-rays diffraction (XRD) and scanning electron microscopy (SEM). Solvent extracts and extract-mediated AgNPs were assessed for their antimicrobial activities using a well diffusion assay. Results: The FT-IR and phytochemical analysis revealed various types of bioactive compounds and functional groups such as tannins, reducing sugars, flavonoids, steroids, polysterol, terpenoids, saponins, glycosides, phenols, anthraquinones, and alkaloids. The nanoparticles synthesized were round and spherical in shape, with a size range of 35–45 nm, as demonstrated by XRD and SEM analysis. All the extracts demonstrated comparable activity against the bacterial strains; however, the methanolic extract proved relatively more potent. Among the panel of multidrug-resistance (MDR) bacterial strains, Staph aureus, Klebsiella, Citrobacter and Shigella proved relatively more sensitive towards all solvent extracts. Likewise, all fungal strains were susceptible towards all the extracts, though Helminthosporium and Trichoderma were relatively more susceptible. The antimicrobial activity of the AgNPs was also assessed alone and in combination with standard drugs. Conclusion: Various solvent extracts of Justicia adhatoda contain numerous bioactive compounds with antimicrobial activities. Both solvent extracts and extract-mediated AgNPs hold broad-spectrum antimicrobial potential. However, the AgNPs synthesized in the aqueous extract were relatively more potent against bacterial and fungal strains. Herein, the potential of Justicia adhatoda as an antimicrobial agent is demonstrated.