Background: The restricted effect, significant toxicity, and emerging resistance of anti-toxoplasmosis synthetic agents impose the search for alternatives. Aim: The current research aimed to evaluate the prophylactic and therapeutic efficacy of Rosmarinus officinalis extracts and their mixtures against chronic murine toxoplasmosis and to clarify the phenomenon of delayed death. Methods: This research included two experimental designs, the first to test the preventive and curative efficacy of the extracts and the second to assess delayed death in mice infected with the ME49 strain of Toxoplasma gondii. The essential oils of the plant were analyzed by gas chromatography/mass spectrometry. Results: Treatment with a mixture of rosemary extracts displayed reduction rates of 81% for T. gondii cyst burden and 23% for cyst viability. The reinfected group with the pretreated cysts reported 93.4% reduction in cyst burden and 95.4% in cyst viability. Moreover, 90% reduction of the infectivity rate was obtained. The therapeutic efficacy of this mixture was superior to its valuable prophylactic effect. Histopathological examination of liver and brain tissue exhibited marked improvement. Both extracts possess free radical scavenging and antioxidant activities evidenced by high expression of iNOS stain. Our results were signified by low BAG-1 gene expression and massive mutilation of T. gondii cyst in the targeted group using scanning electron microscopy. Analysis of R. officinalis revealed the presence of isobornylformate as a novel ingredient. Conclusions: R. officinalis displays a therapeutic rather than prophylactic potential, indicating the emergence of an effective safe alternative therapy.