When Hippocrates said, “All disease begins in the gut,” was he offering timeless wisdom or was it just an ancient claim? What was once, a few centuries ago, a bold statement has today become a well-known fact that modern medicine focuses on. There has been growing interest in the connection between gut microbiome, diet, and mental health in recent years. The mechanisms of gut–brain interactions have been identified as neuronal, endocrine, metabolic, and immune, all of which are affected by diet. This paper aims to present an overview of current knowledge on the interactions between the gut, the brain, mental health, and the effects of diet on these interactions. It examines possible management strategies for dysbiosis and provides an overview of major research gaps and future research directions to enhance understanding of the gut–brain axis. Even though we cannot go as far as to say, as Hippocrates did, that “All disease begins in the gut,” current scientific knowledge enables us to state that “A happy gut is a step towards a healthier mind and body.”