Cattle mastitis is one of the most common problems affecting the global economy. The main objectives of this study were: (i) assessing the antimicrobial activities of essential oils (EOs) from Artemisia annua, Mentha longifolia, and Vitex agnus-castus, and their nanoemulsions, against mastitis-associated pathogens, and (ii) identifying the chemical components of these EOs. Eucalyptol (15.34%), artemisia ketone (9.59%), and endo-borneol (8.24%) were found to be the main components of A. annua EO; pulegone (37.37%), isomenthone (32.03%), and eucalyptol (20.61%) were the major constituents of M. longifolia EO; cuminic aldehyde (66.15%) and 2-caren-10-al (6.65%) were the main compounds in V. agnus-castus EO. The EOs were assessed against the growth of the bacteria Citrobacter diversus, Proteus vulgaris, Escherichia coli, and Staphylococcus aureus as well as the fungi Candida albicans, Candida glabrata, Candida tropicalis, Candida krusei, and Candida albicans. We found that the three EOs were most potent against P. vulgaris, S. aureus, and E. coli,with a significant activity against the other strains as well. The EOs also showed significant antifungal activities against all the five Candida strains, with V. agnus-cactus EO being the most potent. The EO nanoemulsions were found to be more active than the pure oils against all the microbes tested. Our results clearly demonstrate the potential of these EOs and their nanoemulsions as antimicrobial agents.