Arnica montana L. (mountain arnica) is a medicinal plant with diverse biological activities commonly used in pharmacy and cosmetics. The attributes of A. montana are related to e.g., the concentration and chemical composition of its essential oils (EOs). Therefore, the objective of this study was to: (i) characterize the chemical composition of EOs obtained from mountain arnica flower heads, rhizomes, and roots used as a pharmacopoeial material, (ii) demonstrate the effects of particular EO types and their combinations on antibacterial activity, and (iii) demonstrate the effect of the presence of A. montana EOs and their combinations with commercial antibiotics on their antibacterial activity. Essential oils obtained by hydrodistillation from different parts of A. montana were screened for their chemical composition and antibacterial properties. The chemical composition of the EOs was determined using the GC–MS technique. E-caryophyllene, caryophyllene oxide, germacrene D, farnesyl acetate, and dodecanal were the main components of the EO distilled from the flower heads. In turn, 2,5-dimethoxy-p-cymene, 2,6-diisopropylanisole, p-methoxyheptanophenone, and thymol methyl ether were the main molecules detected in the EO from the A. montana rhizomes and roots. The data clearly indicate that the presence of mountain arnica EO alone and in the interaction with commercial antibiotics (amoxicillin, ciprofloxacin, metronidazole) has a beneficial effect on their antibacterial activity.