Concrete contributes carbon emissions mostly due to the cement it contains, which can be partially replaced by supplementary cementitious materials. However, many of these materials may become unavailable as carbon intensive industries, of which they are by-products, are going to be out phased. Ashes from the incineration of municipal solid waste, a process which is implemented to generate energy, should be examined as an alternative and renewable source of supplementary cementitious material. In this study, carbonated municipal solid waste incineration fly ash (MSWIFA) was employed to replace 30% (by volume) of the cement content in cementitious mortars, formulated from regular strength concrete. Two different carbonation methods were used for CO2 embedment in MSWIFA, differing in the method employed to deliver the CO2 to the substrate. In the first, flu ash samples (FA-C1) are made to react with gaseous CO2, while in the second, samples (FA-C2) are reacted with a soluble carbonate salt. Four mix designs were examined, including two reference mixes. For each mix design, 3 specimens under bending, and 6 specimens under compression were tested at 7, 28, and 84 days. Replacement of 30% of the cement with MSWFA results in a decrease in comparable performance with similar mixes containing commercial fly ashes at an equal volume replacement ratio.