l-Theanine (l-THE) is a nonproteinogenic amino acid derived from green tea (Camellia sinensis), which exhibits strong antioxidant-like properties and contributes to the favourable umami taste sensation. Several studies have reported that the consumption of this amino acid has many therapeutic effects, including improvements in brain and gastrointestinal function, cancer drug therapeutic efficacies, antihypertensive effects, and improved immune function. Considering the recent Western commercialisation and popularity of green tea consumption as a nootropic agent in humans, the aims of this review were to consolidate the existing knowledge from ex vivo and in vitro animal models and attempt to highlight the applicability of l-THE towards the human clinical trials. Considering the anti-inflammatory and antioxidants effects of l-THE presented in the current review, further research must translate the existing knowledge gained from animal and cell models to exploring the potential metabolic health benefits and moderating effects on the pathogenesis of conditions such as obesity, arthritis, depression, and type 2 diabetes in human trials. This will bridge the gap in literature and provide more insights into the mechanisms driving pathologies characterised by the inflammatory response and oxidative stress.