Background/Objectives: Carvacrol, a monoterpenoid phenol found in essential oils, exhibits many biological activities, including anticancer properties through mechanisms such as induction of apoptosis. These properties can be enhanced if encapsulated within nanoparticles. This study focuses on producing functionalized carvacrol-loaded nanostructured lipid carriers (NLCs) applied to the treatment of breast cancer. Methods: NLCs were produced by hot emulsification with the sonication method and optimized by the Box–Behnken design, considering Precirol^® (1, 4, 7%), carvacrol (1, 5, 9%), and Tween^® (0.1, 0.5, 0.9%) as independent variables. Results: The optimized NLC containing 2% carvacrol had a particle size of 111 ± 2 nm, PdI of 0.26 ± 0.01, and zeta potential of −24 ± 0.8 mV. The solid lipid (Precirol^®) was the variable that most influenced particle size. NLCs were functionalized with Pluronic^® F68, cholesterol, chitosan, and polyethylene glycol (0.05–0.2%), with oNLC-Chol presenting the most promising results, with no significant increase in particle size (±12 nm) and high encapsulation efficiency (98%). Infrared spectra confirm effective carvacrol encapsulation, and stability tests showed no significant physicochemical changes for 120 days of storage at 4 °C. When incubated with albumin (5 mg/mL), NLCs showed overall good stability over 24 h, except for oNLC-Chol, which increased slightly in size after 24 h. In addition, oNLC increased the cytotoxic effect of carvacrol by 12-fold, resulting in an IC₅₀ of 7 ± 1 μg/mL. Conclusions: Therefore, it was possible to produce stable, homogeneous NLCs with nanometric sizes containing 2% carvacrol that displayed improved anticancer efficacy, indicating their potential as a delivery system.