In this study, we investigated the effect of plasticizers on the mechanical properties of poly(styrene-co-methacrylate) (PSMANa) ionomers with different ion concentrations. The first study was conducted to investigate how the addition of the polar plasticizer glycerol to PSMANa ionomers with different ion concentrations affects the mechanical properties of the ionomers. Glycerol was added to PSMANa ionomers having ion concentrations of 3.1, 6.4, 8.0, 11.9, 15.7 and 22.4 mol% to make a weight % of plasticizer 5, 10, 20 and 30 wt%. The mechanical properties of the plasticized ionomers were measured by DMA. In the experimental results, when looking at the change in Tg depending on the plasticizer content, matrix Tg decreased slightly in the case of ionomers with low ion contents, and cluster Tg decreased sharply with a small amount of plasticizer. On the other hand, in the case of ionomers with high ion contents, matrix Tg was hardly observed, and cluster Tg decreased strongly. Looking at the correlation between the ion content and the plasticizer content, it was seen that the higher the ion content, the more the plasticizing effect is caused by the addition of glycerol. The second study was conducted to investigate how the non-polar plasticizer dioctyl phthalate (DOP) affected the mechanical properties of the PSMANa ionomers with different ion concentrations. The experimental method was the same as the previous method. It was found that both matrix Tg and cluster Tg were decreased as the DOP content increased. Matrix Tg decreased linearly regardless of ion content, cluster Tg decreased linearly with respect to low ion content (matrix-dominant) ionomers, Matrix Tg’s decreased linearly regardless of ion content, cluster Tg’s decreased linearly for low ion content (matrix-dominant) ionomers, and decreased curvilinearly for high ion content (cluster-dominant) ionomers. SAXS experiments were performed for morphological analysis, and as a result, there was almost no change in the DBragg value. It was found that DOP did not significantly affect morphological changes because it caused plasticization in hydrocarbon-rich regions.