A simple cost-effective method (spontaneous emulsification) for fabricating physically stable antimicrobial nanoemulsions from essential oils is described. These nanoemulsions (10 wt % total oil phase) were formed by titration of a mixture of essential oil (carvacrol), carrier oil (medium chain triglyceride, MCT), and nonionic surfactant (Tween) into an aqueous solution with continuous stirring. Oil phase composition (carvacrol-to-MCT mass ratio) had a major impact on initial droplet diameter, with the smallest droplets (d ≈ 55 nm) being formed at 2.5 wt % carvacrol and 7.5 wt % MCT. Surfactant type also had an appreciable impact on mean droplet diameter, with Tween 80 giving the smallest droplets (d ≈ 55 nm) from a group of food-grade nonionic surfactants (Tween 20, 40, 60, 80, and 85). The droplet size also decreased (from >5000 to <25 nm) as the total surfactant concentration was increased (from 5 to 20 wt %). The long-term stability and antimicrobial efficacy of selected nanoemulsions was examined at ambient temperature. The stability of the nanoemulsions to droplet growth during storage decreased as the carvacrol concentration in the oil phase increased. Conversely, the antimicrobial efficacy of the nanoemulsions increased as the carvacrol concentration increased. These results have important implications for the design and utilization of nanoemulsions as antimicrobial delivery systems in the food and other industries. They suggest that the carrier oil concentration must be carefully controlled to obtain good physical stability and antimicrobial efficacy.