Background: Tagetes lucida Cav (T. lucida) is an herbaceous plant from the family Asteraceae, native to Mexico, Guatemala, and other Mesoamerican countries. Their common names are: (i) Spanish names: “hierbanís”, “jericón”, “pericón vomol”, “Santa María”, “cuchrucumín”, “falso hipericón”, “hierba añil”, “pericón”, “periquillo”; (ii) indigenous names: “ichka”, “cuahuyauhtli”, “yahuhtli”, “naná uarhi”, and (iii) English names: “sweet-scented marigold”, “sweet mace”, and “sweet marigold”., Its chemical composition of coumarins, estragole, quercetin, and flavonoids has antioxidant, anti-inflammatory, and antimicrobial effects. This study aimed to evaluate the effect of a mouthwash made with T. lucida for control of chronic periodontitis (CP) in older adults. Methods: We carried out a quasi-experimental study with a convenience sample of 60 older adults with CP. The sample was divided into a placebo group (PG) n = 30, administered with a mouthwash made with alcohol diluted with purified water (10%), and the experimental group (EG) n = 30 treated with a mouthwash made with T. lucida extract in 10% alcohol; both groups were administered three times a day for three months. Results: We found a significant improvement in the EG group compared with the PG group in: probing pocket depth (baseline, 5.2 ± 0.81 vs. post-treatment, 3.1 ± 0.43, p < 0.05), the clinical attachment loss (baseline, 6.6 ± 1.9 vs. post-treatment, 2.1 ± 1.5, p < 0.05), lipoperoxides (baseline, 0.056 ± 0.03 vs. post-treatment, 0.034 ± 0.02 µmol/L, p < 0.05), and chronic inflammation markers (IL-1, IL-8, and TNF-α) (p < 0.05). Conclusions: Our findings suggest that mouthwash made with T. lucida extract, administered three times a day, clinically improved chronic periodontitis in older adults, associated to a significant decrease in lipoperoxides and proinflammatory markers.