Ageratum conyzoides L. (Asteraceae) possesses wound healing, antimicrobial, and anti‐inflammatory activities. Traditionally, this plant is used in skincare. The essential oil (EO) of A. conyzoides aerial parts contains the chromenes precocene I (1) and precocene II (2) as major constituents. This study evaluated precocene II (2) for its in vivo efficacy, mechanistic analysis, and synergistic interaction with norfloxacin against methicillin‐resistant Staphylococcus aureus (MRSA). The study showed that 2 interacted synergistically (Fractional Inhibitory Concentration Index, FICI≤0.5) with norfloxacin and oxacillin and reduced their MIC values significantly. These observations were further validated using the mice model and showed a significant reduction in bacterial load at much lower doses of 2 and norfloxacin without toxicity at 200 mg/kg body weight. Mechanistic studies revealed that 2 regulates bacterial resistance against clinical isolates of S. aureus through membrane disturbance in a dose‐ and time‐dependent manner. Further, precocene II (2) damaged the membrane of the bacteria, as observed from the increased membrane depolarization and uptake of propidium iodide. It also displayed high selectivity towards S. aureus over mammalian cell lines. The in vitro results highlighted the synergistic activity of 2 through the cell membrane damage without any detrimental effect on mammalian cells. In vivo results showed that 2 in combination with norfloxacin significantly reduced bacterial load at much lower concentrations through synergistic interaction without apparent toxicity.