Synthetic oxygen carriers based on geopolymer composites for chemical looping combustion have been recently proposed as alternative to traditional ones. In this work, this novel class of oxygen carriers was further developed by introducing manganese oxide (Mn2O3) as active phase. A geopolymer composite with Mn2O3 powder was produced and compared to a previously developed Fe-based system. Additionally, a mixed iron/manganese composite was fabricated to investigate possible combined effects of the two oxides. Laboratory experiments were carried out in a plant for the combustion of a CO rich gas from char gasification in CO2, consisting of two sequential fixed beds for char gasification and chemical looping combustion. The tests conducted at 900 °C pointed out the excellent performance of the Mn- and MnFe-based oxygen carriers, achieving an efficiency of CO conversion up to 99% during the first minute of operation with gas residence time less than 1 s. Moreover, the Mn-based materials exhibited the ability to release O2 during the initial stage of combustion. In the mixed iron/manganese system, the formation of the mixed phase MnFe2O4 was detected by XRD analysis, resulting in a beneficial synergistic effect. Both developed oxygen carriers (Mn- and MnFe based) exhibited similar CO conversion profiles after repeated cycles of operation.