The performance of a zinc-cerium redox flow battery (RFB) with mixed methanesulfonate (MSA) – chloride negative electrolyte is compared to that of a zinc-cerium RFB with pure MSA electrolyte. Half-cell studies on a polyvinyl-ester carbon electrode confirm that the addition of Cl− ions increases the amount of zinc deposited during cathodic polarization and the exchange current density of the zinc redox reaction. Hence, an electrolyte with a composition of 0.9 mol dm−3 ZnMSA and 0.6 mol dm−3 ZnCl2 in 1 mol dm−3 MSA is chosen as the negative electrolyte for use in a bench-scale zinc-cerium RFB. Reference electrodes are also incorporated in the RFB set-up so that the potential of each electrode can be monitored during operation. The results of the full-cell experiments show that the battery voltage efficiency increases with mixed MSA-chloride electrolyte due to the lower overpotential of the zinc half-cell reaction. Moreover, under the conditions where the zinc redox reaction is the main limiting factor, the charge efficiency rises significantly when the mixed solution is employed. Additionally, a life-cycle analysis of the battery based on a 15-min charge period and current density of 25 mA cm−2 shows that the total operating time and number of cycles increases from 42 h and 97 cycles, respectively, in the case where the conventional negative electrolyte is used to more than 75 h and 166 cycles when Cl− is added. Based on these results, a zinc-cerium RFB with mixed MSA-chloride negative electrolyte has a notably longer life and higher energy efficiency compared to conventional zinc-cerium RFBs.