Dental caries is a bacterial infection caused by Streptococcus mutans, which forms plaque and produces acids that demineralize tooth enamel. The increasing prevalence of caries has encouraged the development of natural antibacterial agents that are safer and more biocompatible than synthetic materials. Lemongrass (Cymbopogon citratus) and basil (Ocimum sanctum) contain bioactive compounds such as flavonoids, alkaloids, saponins, and tannins that demonstrate antibacterial properties and may exert synergistic effects when combined. This study evaluated the inhibitory effect of combined lemongrass and basil extracts on S. mutans and determined their Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) using an in vitro design with serial dilution and colony-counting methods. Cultures of S. mutans in Brain Heart Infusion Broth were treated with extract concentrations of 100%, 50%, 25%, and 12.5%, alongside chlorhexidine and negative controls; data were analyzed using the Kruskal–Wallis test followed by the Mann–Whitney post hoc test. The results showed effective antibacterial activity, with an MIC of 12.5% and an MBC of 25%, and significant differences between treatment and control groups (p = 0.005), although no significant differences were observed among extract concentrations. These findings indicate that combined C. citratus and O. sanctum extracts possess synergistic antibacterial effects and hold potential as natural agents for the prevention of dental caries.