This study investigated the effects of non-thermal atmospheric plasma (NTAP) treatment on the growth, chemical composition, and biological activity of geranium (Pelargonium graveolens L’Herit) leaves. NTAP was applied at a frequency of 13.56 MHz, exposure time of 15 s, discharge temperature of 25 °C, and power levels (T1= 50, T2 = 80, and T3 = 120 W). Results demonstrated significant increases (P < 0.05) in fresh and dry biomass at all treatment levels compared to control, with the highest improvements seen in T3. Mineral content (K, P, Ca, Fe, Mg, Zn, and N) was significantly elevated, particularly at T3. Chlorophyll content (a + b and carotenoids) also showed marked increases across all treatments, correlating with enhanced photosynthetic rates. Improved photosynthesis led to enhanced accumulation of primary metabolites, such as amino acids, organic acids, and fatty acids. NTAP treatments, mainly T3, significantly increased levels of essential and non-essential amino acids, oxalic, isobutyric, and fumaric acids. They also enhanced unsaturated fatty acids, such as oleic acid (C18:1), and saturated fatty acids, including myristic (C14:0) and stearic (C18:0). These improvements provided precursors for the synthesis of secondary metabolites, particularly phenolics. The increased phenolic content in turn explained the improved antioxidant capacity observed in Fluorescence Recovery After Photobleaching FRAP, anti-lipid peroxidation, superoxide radical scavenging, and hydroxyl radical scavenging assays, especially at T2 and T3 treatments. Antimicrobial activity was elevated across all treatments, with the T3 treatment notably inhibiting all tested bacterial and fungal strains, particularly Sarcina lutea.