Non-structural carbohydrates (NSC) are key components of plant tissue being involved in several metabolic and physiological processes, greatly affecting the growth and adaptation of plants. The total NSC content varies with species, organs, seasons, and environmental conditions. In this study, we compare the seasonal dynamics of NSC, including total soluble sugars (SS) and starch (St), present in leaves of Pinus pinaster, a tree native to the Western Mediterranean basin, and the invasive Hakea sericea, particularly problematic in areas with Mediterranean-type of climate. With no exception, SS, St, and NSC contents changed throughout the growing season in both species but with distinct patterns. In the native species, the highest SS and NSC contents occurred in winter, while St content was higher in spring. In turn, Hakea sericea had higher SS content in summer, and higher St and NSC contents in winter. Our results also showed that St was the dominant form of NSC. Comparing both species, Hakea sericea accumulated significantly higher amounts of St and NSC than Pinus pinaster, especially in winter. By contrast, the SS content did not vary significantly between species. The observed differences in NSC accumulation seemed to be related to the phenological cycle of both species but also suggest the superior performance of Hakea sericea over Pinus pinaster, even under unfavorable environmental conditions, which may explain its invasion success in the Mediterranean.