The chemical composition and antimicrobial activity of essential oils (EOs) obtained from three medicinal plants of the Moroccan flora were evaluated. The chemical composition of EOs of Thymus leptobotrys, Laurus nobilis and Syzygium aromaticum was determined using a gas chromatograph coupled with mass spectrometry. Carvacrol (75.05%) was the main constituent of T. leptobotrys EOs, while 1,8-cineole (31.48%) and eugenol (82.16%) were the predominant components of L. nobilis and S. aromaticum EOs, respectively. The antimicrobial activity of the EOs was evaluated qualitatively and quantitatively against 18 microbial strains pathogenic to humans by using the disc diffusion method, and by measuring the minimum inhibitory concentration (MIC) and minimum microbicidal concentration (MMC). The EOs of T. leptobotrys were the most active against the strains tested, with inhibitory zone values ranging from 7.00 to 45.00 mm, and MIC and MMC values ranging from 0.312 to 80.00 mg/mL. In many cases, these EOs exhibited higher antibacterial and antifungal activities than the chemical compounds ciprofloxacin and fluconazole, respectively. This high antimicrobial activity can be ascribed to their richness in carvacrol. The EOs of T. leptobotrys, L. nobilis, and S. aromaticum could be considered a promising alternative to replace chemical antimicrobials, and a readily available natural source of bioactive compounds.