This article deals with the effects of using COOH functionalized multi-walled carbon nanotubes (MWCNTs) on mechanical properties and durability of cement mortars. In this study, the behavior of nanocomposites modified with well-dispersed MWCNTs at 0.05, 0.1, 0.2, 0.3, and 0.4 wt percent of cement using compressive and flexural strength tests subjected to environmental conditions of combined sodium-magnesium sulfate and limestone-saturated water up to long-term exposure of 960 days were analyzed. The results revealed that adding functionalized MWCNTs as a bridging factor increases the flexural strength of nanocomposites stored in limestonesaturated water and sulfate solution by 37% and 42%, respectively. Moreover, as a filler, by refining pores and changing the microstructure, it improved compressive strength in the two mentioned environments by 25% and 39%, respectively.