This review explores the multifaceted roles and transformations of melatonin across the animal and plant kingdoms. In animals, melatonin is well-known for its regulation of circadian rhythms, influence on mood, and role in various physiological processes. Recent research has highlighted its potential in treating mood disorders, particularly Seasonal Affective Disorder (SAD), although results remain mixed, indicating a need for further investigation. In plants, melatonin has emerged as a crucial molecule involved in growth regulation, stress responses, and overall plant health. This review provides a comprehensive comparison of melatonin’s biosynthesis, biological functions, and practical applications in both realms. By examining its impact on health and development in animals and its role in enhancing growth and stress tolerance in plants, this review highlights both the commonalities and unique aspects of melatonin’s function across these kingdoms. Furthermore, the potential applications of melatonin in medicine and agriculture are discussed, reflecting on future research directions and the broader implications of this versatile molecule.