The durability performance of sustainable concrete that included reactive magnesium oxide (MgO) and fly ash (FA) was assessed in this study. Due to the sustainability advantages and decreased shrinkage, the partial substitution of cement with these two materials is an intriguing alternative for the building industry. The percentages of FA incorporation by partial cement replacement were 0%, 15%, and 30%. MgO was added to concrete at different rates: 0%, 5%, 10%, and 20%. There were two different kinds of MgO utilised, one from Spain and the other from Australia. A comparison of these two materials' individual incorporations was made, and subsequently a comparison of their simultaneous usage was made. In all tests (water absorption through capillarity and immersion, carbonation depth, and resistance to chloride penetration), performance reductions ranging from 3% to 95% were found. Due to the gradual hydration of these two alternative materials, the performance gap between them and the reference concrete tends to narrow with time. It was discovered that the adverse effects did not overlap in the majority of the experiments. In other words, the performance losses brought on by the simultaneous integration of MgO and FA were less than the total of the losses brought on by each component's incorporation separately.