This study investigated the durability performance and microstructural changes of fly ash–ground granulated blast furnace slag (GGBS)-based geopolymer mortar (GPM) made with different sand-to-binder () ratios and subjected to sulfate and acid environments. The durability performance of GPM in terms of visual observation, change in weight, and change in compressive strength was investigated after exposure to 6% , 6% , , and solutions for a period of 26 weeks. The weight gain percentage of the GPM mixes exposed to sulfate solutions decreased with an increase in GGBS content, whereas the weight loss percentage of the mixes exposed to acid solutions increased with an increase in GGBS content and ratio. GPM made with lower GGBS content, i.e., 15%, had the maximum gain and the minimum loss in compressive strength when exposed to sulfate and acid solutions, respectively, which is in line with the variations in peak intensity of aluminosilicate gels from X-ray diffraction (XRD) analysis, and elemental ratios obtained from energy dispersive X-ray spectroscopy (EDS) analysis. GPM mixes made with a higher ratio (2.5) performed better in offsetting the loss in compressive strength in sulfate and acidic environments. The results of XRD, EDS, and field emission scanning electron microscopy (FESEM) analyses showed more formation of gypsum in GPM made with a higher GGBS content when exposed to sulfate and solutions. The atomic Na/Si ratio, Al/Si ratio, and Ca/Si ratio obtained from EDS analysis indicated comparatively better stability of geopolymer gels in the mortar made with lower GGBS content when exposed to acid solutions.