Phenotypic flexibility may facilitate range expansion by allowing organisms to maintain high levels of performance when introduced to novel environments. Phenotypic flexibility, such as reversible acclimatization, permits organisms to achieve high performance over a wide range of environmental conditions, without the costly allocation or acquisition tradeoffs associated with behavioral thermoregulation, which may expedite range expansion in introduced species. The northern curly-tailed lizard, Leiocephalus carinatus, was introduced to the USA in the 1940s and is now established in southern Florida. We measured bite force and the thermal sensitivity of sprinting of L. carinatus during the winter and spring to determine how morphology and performance varied seasonally. We found evidence of seasonal variation in several aspects of physiological performance. Lizards sampled in spring sprinted faster and tolerated higher temperatures, while lizards sampled in winter had high performance over a wider range of temperatures. Furthermore, seasonal differences in physiology were only detected after generating thermal reaction norms. Both sprint and bite force performance did not differ seasonally when solely comparing performance at a common temperature. No seasonal relationships between morphology and performance were detected. Our results suggest that L. carinatus may use reversible acclimatization to maintain high levels of performance across seasons not typically experienced within their native range. Thermal physiology plasticity may ameliorate the impacts of sub-optimal temperatures on performance without the cost of behavioral thermoregulation. Our work highlights the importance of utilizing reaction norms when evaluating performance and the potential ecological impacts of introduced species.