Efficient and robust synthetic processes of active pharmaceutical ingredients (APIs) are highly desirable, and continuous flow chemistry is a critical component of this endeavor. The clinical importance of ondansetron, a World Health Organization essential medicine, prompted us to investigate continuous synthetic approaches to this API. Our efforts to improve the synthetic processes led to a continuous condensation step and a continuous Mannich reaction. A continuous work-up and purification process was also established for the former. A batch process was employed for an elimination and Michael addition step, as it not only accommodated the physical properties of the reaction mixtures, but also provided a high productivity of the desired product. Taken together, these findings demonstrate the complementary advantages of flow and batch chemistry in API synthesis.