Myeloperoxidase (MPO), an abundant protein in neutrophils, monocytes, and macrophages, is thought to play a critical role in the pathogenesis of various disorders ranging from cardiovascular diseases to cancer. We show that mesna (2-mercaptoethanesulfonic acid sodium salt), a detoxifying agent, which inhibits side effects of oxazaphosphorine chemotherapy, functions as a potent inhibitor of MPO; modulating its catalytic activity and function. Using rapid kinetic methods, we examined the interactions of mesna with MPO compounds I and II and ferric forms in the presence and absence of chloride (Cl-), the preferred substrate of MPO. Our results suggest that low mesna concentrations dramatically influenced the build-up, duration, and decay of steady-state levels of Compound I and Compound II, which is the rate-limiting intermediate in the classic peroxidase cycle. Whereas, higher mesna concentrations facilitate the porphyrin-to-adjacent amino acid electron transfer allowing the formation of an unstable transient intermediate, Compound I*, that displays a characteristic spectrum similar to Compound I. In the absence of plasma level of chloride, mesna not only accelerated the formation and decay of Compound II but also reduced its stability in a dose depend manner. Mesna competes with Cl-, inhibiting MPO's chlorinating activity with an IC50 of 5 μM, and switches the reaction from a 2e- to a 1e- pathway allowing the enzyme to function only with catalase-like activity. A kinetic model which shows the dual regulation through which mesna interacts with MPO and regulates its downstream inflammatory pathways is presented further validating the repurposing of mesna as an anti-inflammatory drug.